Windows 7 (64-bit) on two machines, Ubuntu 11.10 (64-bit), Windows XP (32-bit), Ubuntu 10.04 (32-bit), Linux Mint 12 (32-bit) and EasyPeasy 1.6 (32-bit).
Windows because most of my software runs on Windows and various Linux distros because I can't decide which is better, and I like open source. EasyPeasy because I'm too lazy to do anything about it, originally tested it for a friend of mine who's part of the development team (though I think they've stopped working on it).
__
Oh, and if we're doing previous systems, my first computer ran on MS-DOS. Then I upgraded to Win 3.11, and then another Win 3.xx as far as I recall. Tried Windows 95 when it was released, didn't like it and it kind of lagged - yay - so I went back to Win 3.11 or something. I skipped 98 altogether myself and got Millenium (or Windows Me), which was absolute shite, but I didn't really use computer all that much any more in that period. Upper Elementary(?) and High School was serious business a party hard affair, and I hardly spent time home at all. At some point in-between Elementary and College, though, I got myself XP. Then I got Vista after I started studying at University six years ago. Finally getting Windows 7 back in April, last year. Altogether, I most fondly remember MS-DOS. I disliked XP so bad, ugliest OS Microsoft ever did. As for Linux, I've encountered Red Hat before, but I started using Ubuntu 7 as my first open-source OS. I used it purely in Terminal because, you know, MS-DOS nostalgia.