One interesting thing about the software world, I find, is that there is a trickle-up system for innovation. Things like live searches, better task bars, and the much-debated widget/gadget/dashboard idea often have had their seeds in smaller OSes or pieces of software. Designers will find a way to work something in their program that others may be able to adapt to something bigger, and thus the snowball begins to grow.
Some of the OSes (perhaps more accurately called Desktop Environments, says a commenter) on this interesting list may be familiar to you, some not, but doubtless all are interesting and relatively unique in their strengths. I’m far from an expert in this field (in fact, it would be a stretch to call me a dabbler), but I think I have the appreciation for usability and good design that comes from a lifetime of using computers and technology of all kinds.
