Like a love child of eBay and CouchSurfing.com or craigslist, AirBnB lets anyone that owns space fit for accommodating travelers – be it a couch in a small apartment or the master bedroom of a 19th-century castle – post that space as a listing on its website and connect potential renters to its respective owners.

Once called Airbed and Breakfast in full, AirBnB aims to provide a secure online marketplace for these types of transactions and make people ‘travel like humans’ again.

Now the startup, which came out of Y Combinator’s winter 2009 class, is making it easier for people who share interests (e.g. photography) or have another connection with each other (e.g. alumni who’ve attended the same university) to share travel accommodation and recommendations thanks to the addition of Groups.

This past weekend, Twitter co-founders Evan Williams and Biz Stone spoke at Startup School. Before they took the stage, they met up with Y Combinator’s Paul Graham and came up with a great idea: A deal to ensure that Y Combinator startups working on Twitter-related projects have priority access to the tweet stream, as well as access to Twitter’s team.

The idea led Graham to delay the application deadline for YC Winter 2010 startups for two days, so they could release two new Requests For Startups (RFS), YC’s recently announced program that gives applicants basic big picture ideas from which to form startups around. One of these new RFSes is obviously to build something on top of Twitter. Here’s the description:

Jason Fried of 37Signals has taken the stage at Y-Combinator’s Startup School this morning at UC Berkeley. I’m taking my notes below on his talk.

Fried has started off by talking about bootstrapping startups. Startups that bootstrap are more “money hungry” then companies that are funded. If you are a funded company, you generally have money to spend that investors encourage you to spend as well. If you’re a bootstrapped company, you’re hungry to make money.

Now that Windows 7 is being met with mostly positive reviews and green-lighting by software professionals, no doubt many will be making the switch to Microsoft’s new operating system in the next couple of weeks. Which is a bit of a pain if you stuck with Windows XP and skipped Vista altogether, because then the best thing for you to do is perform a complete clean install after backing up your data and programs.

Wiping out your data is one thing, but you’ll also need to install all your applications from scratch again once you get Windows 7 up and running on your computer.

Social shopping is a nut that no one has been able to crack. The idea sounds good; people enjoy shopping together in malls and such. But online, there have been many attempts, and many failures. Plurchase, a Y Combinator-backed company, is the latest to take on the task.

Obviously, Plurchase thinks it has what it takes to succeed in the space with a new method of doing things. In the past, social shopping sites have either required you to have a browser plug-in to converse with friends while on shopping sites, or they have been entirely new sites that everyone had to go to. Plurchase simply offers a social overlay to the sites you already use, and makes it easy to invite friends to join you.

I’m here at Y Combinator’s Demo Day, where the latest batch of the incubator program’s startups are showing off the fruits of their labor to a room of press and VCs from around Silicon Valley. The turnout today is huge — VCs representing billions of dollars in managed funds are here, with investors from US Venture Partners, XG Ventures, Founder’s Fund, Greylock Ventures, First Round Capital, Bessemer Venture Partners, True Ventures, Freestyle Capital, Venrock, O’Reilly AlphaTech Ventures, FBfund, renowned angel investor Ron Conway and more in attendance. There are also plenty of executives from established companies, including Google exec Bradley Horowitz and Don Dodge from Microsoft. Past Demo Days have certainly received their fair share of attention, but it seems like YC’s larger class size, and perhaps the companies themselves, are bigger draws than ever.

Keep reading for a description of the startups that presented, as well as links to our past coverage of many of them.

The ability to chat live with visitors is an incredibly useful marketing tool for site owners. Whether a site owner operates an e-commerce or consumer or business service site, live chat is increasingly becoming the norm for engaging with visitors in real time. Olark, a Y Combinator-funded startup (re)launching today at Demo Day (it was previously branded hab.la), has made the ability to embed chat into a site incredibly simple.

For free, site owners can embed the widget into their site with just a few lines of javascript. One useful feature that Olark offers is the ability to add one-to-one chat to any website without editing any code. Olark’s short link service lets website owners create a link to any site they would like to chat with a visitor about. When a customer/user clicks on the link, the Olark widget will float over to the visited webpage. The site owner can use an existing IM client, such as GChat or AIM, and each customer will show up in the buddy list. So site owners can interact with visitors from their preferred chat program. Visitors show up on a website owner’s buddy list as soon as they hit the site (with an anonymous ID like Web User 1), with the name of the page they’re on. Basically, you can watch everyone’s progress through the site even when they don’t talk to you.

It is said that a picture is worth a thousand words. Translated into this Twitter world we now live in, that’s like, thousands of characters. Thousands easily beats 140, so the people constantly complaining about Twitter’s brevity need to check out DailyBooth.

What is it? It’s a Twitter-like quick message service, only the main form of communication is pictures. You can send pictures of anything you want, though most users tend to send images of themselves, photo booth-style, and attach messages to them. The result is a crazy amount of interaction in the community. And interestingly enough, unlike Twitter, it seems to be a hotbed for teenage activity.

Y Combinator sees no shortage of startups that apply to be a part of their funding cycles. But they don’t always see all the ideas they’d like to see come out of the classes. So starting with the upcoming Winter 2010 cycle, they have a new idea called RFS, Requests For Startups. Basically, Y Combinator will issue some ideas of what they’re looking for in any cycle, and will accept the ones that pitch the best way to do the idea.

Now, to be clear, Y Combinator will not be forgoing its usual method of combing over any and all startup pitches outside of the ones they lay out. “We don’t expect responses to RFSes will ever be more than a fraction of the applications we accept. We wouldn’t want them to be. Most good ideas should be ones that surprise us, not ones we’re waiting for,” Paul Graham writes on the site today. The hope is that this will help guide some new startups without solid ideas in the direction of something that is missing in the market. Or encourage ones that already have a similar idea to apply.

Y Combinator’s RFSes won’t describe exactly what Y Combinator is looking for, but rather will give a general idea, with the hopes that the startups can come up with even better plans than Y Combinator is thinking of, Graham says.

When most people are faced with the task of building their resume, they fire up Microsoft Word, trudge through a few generic looking templates, and export their page to HTML. Usually this results in something that’s either boring, weird looking (because of formatting issues), or just plain ugly. JobSpice, a new startup that’s launching tonight, is looking to help users build web-friendly resumes that are as good looking as they are easy to customize.

JobSpice comes with a good pedigree: it is part of the latest batch of Y Combinator funded startups, and is co-founded by Andrew McCollum — a Facebook co-founder who served as the social network’s original designer.

Of course, there are already plenty of ways to build your resume — aside from Word, there are a number of online services that will do it for a price, though founders McCollum and Dane Hurtubise say that these generally can be pricey, going for upwards of $100.

In contrast, JobSpice is free, and it goes a long way toward making resumes visually appealing with a minimum amount of effort.

“Whoever manages to change the nature of content display on the Web from a search problem to a recommender problem will reap tremendous rewards.”

That quote, by Greg Linden, the man behind Amazon’s recommendation system, is the dogma of Directed Edge, a new Y Combinator-backed startup in the recommendation space. Amazon, of course does product comparisons, but there’s no reason recommendations shouldn’t be a part of news consumption, music consumption, social networking, basically everything we do on the web. And while there are no shortage of companies out there that focus on some of these different fields specifically, Directed Edge has developed a system that can be plugged into all kinds of different sites.

New Y Combinator funded startup Listia launched this morning, and I already love it. It’s a really smart way to give stuff away for free.

We tend to give away a lot of the random stuff people send us at TechCrunch. Some of it is trivial – stickers, tshirts, etc. But sometimes it’s decent electronic stuff – we get a lot of iPod touches with demo apps to review, for example, and usually the company doesn’t want them back so we give them away to people. Earlier this week I gave away a wireless Google-branded mouse on Twitter, and pondered setting up a Twitter account just to give away all the test stuff cluttering our office.

Often people list free things on Craigslist or donate it to Goodwill, charities or churches.

But Craigslist isn’t a perfect solution, since you will get a mass of emails and need to deal with everyone. And often the winner doesn’t show up. And most charities and churches aren’t really interested in getting actual stuff that they then need to sell for cash. Listia makes it all a lot better.

Every day, sports fans congregate at their nearest big-league stadium to bask in each other’s cheers, body paint, and beer, relishing their shared enthusiasm for the teams they love. But then the game ends. Everyone is forced to head back to their normal lives to trudge through their day jobs once more, at least til the next home game comes round. Social media can offer them some comfort, giving them a chance to take the community home with them. Unfortunately, many professional sports teams are still failing to tap into this effectively.

FanChatter, a Y Combinator funded company that’s launching today, is a startup that’s looking to help fix this problem. The company is focused on helping major sports teams increase engagement both during and after games using user-submitted content, Twitter, and other social services. And while the company is still quite new, it’s already got some major customers, including the Minnesota Twins, the Minnesota Timberwolves, and the University of Oaklahoma.

Live video monitoring systems are great — except for the fact that someone actually has to be watching for them to be of much use. Even if that doesn’t mean watching it live, that means pouring over hours of footage that likely contains a lot of the same thing: Nothing. And that’s why HighlightCam is cool. It condenses video down to just moments where there is actual activity.

It does this by watching for changes in video streams, which is to say that it looks for motion happening within the video. When it senses something, it knows to remember that portion of the video. And, if you set it up to do so, it can also send an email to you to let you know there is activity currently happening on the video stream. You can probably dream up a bunch of uses for something like this, but the obvious ones are monitoring babies from other room, watching your pets when you’re away, and yes, watching to make sure no one is breaking into your house.

It’s been a long time coming, but the computer industry is finally making headway in the switch from old-fashioned, platter based hard drives to the solid state drives (SSDs) found in iPhones and many other electronics. The benefits are obvious: unlike their platter-based counterparts, SSDs have no moving parts, and they’re also significantly faster for many tasks. Unfortunately most software, both server-side and otherwise, has been optimized for the older drives and the physical limitations that come with them. RethinkDB is a new startup that’s looking to capitalize on this problem by building a storage engine for mySQL databases that’s fully optimized for SSD drives, bringing with it large speed boosts and a number of features sure to catch the eye of many developers.

The company, which is part of the latest batch of Y Combinator-funded startups, is in fairly early stages (it started developing the product only two months ago), but it’s already making some substantial headway in the features it can offer.

One of the most key steps to building a succesful startup is figuring out what works — and what doesn’t. To do this, many companies rely on things like A/B testing to figure out which workflows and designs work best. But there are some things that are a bit trickier to measure, like exactly which features your users are taking advantage of, and how they’re using them. Mixpanel, a Y Combinator company launching today, is looking to solve this problem by offering companies a suite of analytics tools that go well beyond what tools like Google Analytics measure.

Founder Suhail Doshi says that most significantly large companies, like Slide for example, have entire teams dedicated to tracking user behavior. If you throw a virtual sheep at your friend using one of Slide’s apps, you can be sure that every element of that action — including who the sheep’s receipient was, the time you threw the sheep, and even what prompted you to throw the sheep in the first place — is being tracked.

Picurio, a Y Combinator startup from its Winter ‘09 session that makes photo sharing between large groups incredibly easy, has taken the beta label off. While there are an innumerable amount of photo sharing sites and applications out there, Picurio is focused on making photo sharing simple and user-friendly between groups of people.

Picurio’s site has a similar user interface to Apple’s iPhoto, except Picurio is on the web and cloud-based. The idea behind Picurio is that you upload photos to a “room” (which has around 2 GB of storage) where you can then create subfolders of different groups of photos and then invite as many people as you want to see the photos. In order to allow others to see the site, you send them a link, (that can be password-protected for privacy) and then they can upload photos of their own to the “room.” As a user, you can share some collections of photos with certain friends and share other rooms with a different set of friends.

With the latest Harry Potter film already setting box office records, there’s no doubt we’re about to see a big surge in the public’s interest in witchcraft and wizardry. Unfortunately, Harry Potter fans will find a limited selection of wizard-centric apps the App Store: there’s currently only one official app available, and it’s mostly a promotional vehicle for the movie rather than an engrossing game. Fortunately, there are a handful of other wizard apps on the marketplace, and one of them — a free game called Magic Wars(iTunes Link) — may be exactly what you’re looking for.

The game was put together by pocketfungames, a Y Combinator startup that specializes in iPhone games. The company originally planned to focus on dating-based applications, but has since shifted gears, looking to take advantage of the fact that many iPhone games still fail to leverage the phone’s Network Effect and integrate social features. In some senses this is a return to the company’s roots, as it was was previously a developer of social games on Facebook.

Last summer, I wrote a lengthy rant against business cards, calling them “virtually useless as one of the last bits of information that we pass non-digitally”. Ten months later, I couldn’t agree with myself more. Just like the handshake, the business card is an annoying relic of the past. Each card I get joins a ridiculously large pile on my dresser that has become effectively useless (the only exception are the cards made out of beef jerky). Sure, I could get a business card scanner and digitize them, but the point is that I shouldn’t have to.

Bump Technologies is a mobile application developer that’s looking to offer a solution, allowing users to transfer their contact information simply by tapping their phones together. And things are looking bright for the small startup — the company has just raised a seed funding round from Y Combinator, and is quickly building up a sizable user base.

Cloudkick, a Y Combinator startup that offers a free server management system to businesses whose web infrastructure is maintained by Amazon’s EC2 or Slicehost cloud-based servers, is unveiling a nifty feature today at Under the Radar. Cloudkick has added the ability for users to migrate their Amazon Machine Images (the template for servers on EC2) on their EC2 servers to another service provider, like Slicehost (which is owned by Amazon Web Services competitor Rackspace). This lets users who are tied to Amazon’s servers be able to easily switch to a less-expensive provider.