BeOS Pro 5.0
RedHat 6.2, SuSE 6 (I think)
SuSE 7.2 Server (Best Linux I've seen)
DOS 6.2, Win 3.1, 95, 98, ME (don't mean to, honest), 2000
I had to port some apps from DOS to Linux a while back, but now don't use DOS really. The old Linux's were not very good so I have them because I horde junk. SuSE 7.2 is running duel boot with 2000 on a couple machines and I love that combo. BeOS was fun for a while, but it is pretty much dead for software.
At the office I sometimes play with Win XP Pro, but I can't say I use it regularly.