Windows Vista (holds my household record of number of crashes and days not working) and Mac OS X Snow Leopard. When I get some free time, I will look into Linux, either Mint or Ubuntu.
I'm currently a disgrace to my own name. My Toshiba won't work with the Linux kernel; it's some problem with the BIOS.
On the plus side, Windows 7 is a big step up from Vista, and I genuinely don't mind it very much. Even if I had a version of Linux installed, I'd be using 7 mostly anyways for school, so it's not a big deal I guess.
I'm running Windows Vista 64bit on my laptop and the main desktop. The wife is running Windows 7. I'm thinking of getting one of those iPad clones that run Android. I've got Android on my HTC Desire phone and I reckon it's great OS for a basic multimedia pad. I don't use 90% of the stuff my laptop is capable of doing so it seems a waste of money to pay for it all.
Primarily Linux and OS X, although I run into Windows from time to time. For anyone non-techie but curious, the latest Ubuntu (installed on a trial-sized partition via Wubi) is about as idiot-proof as it gets which, ironically, is almost a ding in the Linux community.
My desktop is currently running Windows 7 64-bit and my laptop is currently running Ubuntu 10.10. Ideally, I'd be running Ubuntu on both, but I have a couple of gadgets that need Windows-based software to be used most effectively.