Virtualbox.org has just recently posted version 2.0 of it’s virtualisation software!
For quite some time now I have been using Virtual Box, a free opensource alternative to virtualisation from Sun Microsystems who also contribute heavily to Open Office. Virtual box is available for Mac, Windows and Linux.
I am also the owner of some other virtualisation software solutions for the Mac. I have paid for copies of both Parallels and VM Ware. However, I have found myself increasingly using the open source solution. It may not be quite as a polished or integrated as the other solutions, but coupled with open source operating systems (Ubuntu, Gos, OLPC and others) it gives you the flexibity to try different OS’s and software services, and also to pass them on to others. What’s even better is that you can try out open source alternatives for free, on your windows machines without having to partion disks, dual boot etc … just before you make the switch 
BTW it also runs Windows XP at very, very acceptable speeds!