Ubuntu 6.10 (Edgy) on my server, but also Windows 98 because the Linux version of my Jedi Academy server won't run right on Linux
Ubuntu 7.04 (Feisty, Developer preview) on my desktop, and it's great
but I haven't gotten the desktop effects to work (ATi needs to do a lot of work to there cheap junk fglrx package...it supports about...nothing) and also have Windows XP with the Vista Transformation Pack on it as well.