Virtualization software for a Mac

On my PC running Windows 7 I use VMware Player to run Ubuntu in a virtual machine.

What would I use in my MacBook Pro to run Ubuntu? What about for running Windows?

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.

Parallels is one choice. VMWare Fusion is another. I prefer VMWare Fusion because in my experience it's a lot more stable then Parallels.

This Superuser question compares the 3 options mentioned above; Paralles, VMWare Fusion, and Virtualbox. Well worth the read.