I discovered VirtualBox a few months ago and am now migrating to it away from VMWare Workstation. It's stable and performs well.<p>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.<p>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.