I use OS X for browsing the web, web-development, graphic design, video editing, and writing.
I use Windows 7 for music production and playing video games. At some point I want to start dabbling in game development, in which case I'll probably go with Windows for that, too.
I ssh into various Linux servers privately and at work. In my spare I sometimes play around with Linux distros on my desktop just to learn about the current state of affairs. I usually can't see any advantages in it over OS X other than the fact that's it's free software and that it runs on cheap hardware. I keep being curious though.
I don't use any virtual machines because I dislike the sluggishness and I don't really need them.
Sometimes I wish I could get by with only one OS without feeling crippled in some respect. My dream setup would probably be an OSS system that's great with multimedia stuff and has about a 99% adoption so hardware would be supported really well.
But that thought depresses me because it reminds me of the state reality is in, so I try not to have it.