Some time ago I asked for virtual machine advice. Having read the comments and other stuff on various blogs and sites I opted to try VMWare, VirtualIron and VirtualBox.
Here we are nearly a year later and I have to say that (IMHO) Bang for the Buck goes to...drum roll VirtualBox!
The basic edition is open source, well supported and just plain works. A small download of less than 30 MB will get you running.
It isn't a bare metal install, but on my testing with Linux as a host OS it screams running XP and OpenSolaris. I have an OpenBSD VM set up but haven't gotten X to work yet. Installing the guest extensions gives you all sorts of cool stuff.
Eleventy thumbs up!