I discovered VirtualBox a few months ago and am now migrating to it away from VMWare Workstation. It's stable and performs well.
It also offers the Parallels-like feature which allows you to run application windows from your VM in your host desktop environment. This is very handy for doing cross-browser web app testing, where you can (for example) put a Win IE7 browser window right next to your Linux Firefox one to compare things side by side.
I just set this up over the weekend. I hadn't heard of it before and was a little skeptical but it's a lot like the VMWare I've used at work.
It's performing very well and offered some pretty flexible install options (I had an iso file I was going to burn until I noticed VirtualBox could just use it directly).
The thing I really enjoy about VirtualBox is that it just works. I have been using it to run XP under a linux host for things like Skype, and it's really nice not to have to fight with configs and such. Hopefully Sun lets them keep the same ethos after the acquisition.
It also offers the Parallels-like feature which allows you to run application windows from your VM in your host desktop environment. This is very handy for doing cross-browser web app testing, where you can (for example) put a Win IE7 browser window right next to your Linux Firefox one to compare things side by side.