Firefox 3 was a pretty historic release this week, but I'd say that Wine 1.0 might actually beat it -- the open source non-emulator (Wine, after all, Is Not an Emulator) for Windows finally reached their first stable release. And Darwine, the OS X-rated version of Wine, also got a shiny 1.0 designation as well. It still won't work exactly perfectly (you've got to have XQuartz installed, and as with all emulators, there are so many different systems trying to talk to each other that you're bound to run into problems when one of them wants to do something complicated), but for standard Windows apps (Solitare and Spider Solitaire, we're told, work beautifully), it'll do ya.
Of course, we have no idea why you'd want to run anything Windows (ahem), but we won't judge. It's your computer: do what you like.