I have used VMWare pretty extensively since they made it free, and also Microsoft VirtualPC. This is a class of software that the average computer user doesn't really need often, but maybe they should. I like the Browser Appliances that people make up for VMWare for browsing questionable sites. It really does protect the host PC unlike anything else. There are many other uses, I use VMs for testing software I don't really trust.
VirtualBox, I almost passed over today thinking "Eh, I have VMWare". I am glad I didn't. First, its very light weight compared to the others. Second, it has some features the others don't. Nice ones too! Like the arbitrary sizing of the window and guest display settings. Really detailed USB support is another. Also, folder sharing with the host without setting up networking.
I am very impressed with this application.
Edit: Oh, I forgot to mention one of the best features. The ability to do multiple snapshots of the guest and restore any of them independent of the others. THIS feature I intend to test this weekend. If it works as advertised, that would be great.