XP SP3 on my main, and Win7 on a secondary hard drive. i switch from time to time, but seeing as i had XP on the comp WAAAAAAY before i decided to install 7, XP is pretty much the OS i have everything installed on.
@L1D3N: Well you should. I had Vista 64 Ultimate. Windows 7 Ultimate is FAR better in performance and reliability than Vista is. You won't regret it. I was really not "WOWed" by Vista as ol Bill said I would be.
I'm still using XP. I had planned on sticking with it until Windows 7 dropped and, thus, avoid the hassle of Vista. Now that Windows 7 is out, I'll switch to it when I upgrade my system again next year.