"VirtualBox is free (as in beer and as in speech), and I've run Ubuntu under it just fine. Of course, Oracle just bought Sun, so we don't really know yet if they will continue offering VirtualBox for free. Supposedly Windows runs under it just fine too, I haven't tried it, though VirtualBox doesn't offer any sort of video acceleration so lots of the new eye candy in Vista and Windows 7 won't work so well.
The other two common choices are Parallels and VMWare Fusion. I'd check a recent comparison of all three that was published in Macworld for information on how to choose between them."
