Sun bought the VirtualBox open source virtualization project a while back. The project just released a maintenance update (version 1.6.2) today. It can be found at...

VirtualBox

I installed it on a Mac running OS X 10.5 (Leopard) and then installed Microsoft Windows 2000 Professional SP3 as a Guest OS in a virtual machine. Some activities (especially disk based ones) seem a lot slower compared to Parallels Desktop or VMware Fusion. But, the price is right and it runs well enough to test a bit more.

Note that the distributed executable binary installers are free only for personal and evaluation purposes according to the notice on Sun's website.