I put windows 98 on mine just for kicks. I'm also putting XP and I'm going to try ubuntu to see if it works. I use it just because I'm too lazy to put another partition on my hard drive and try dual booting.
I'm doing that. I'm going to test AV software and stuff, basically do stuff that I wouldn't normally do to my OS. I might sound weird but I've always found it fun to play around with security software and see which works and what does.