Check out this review of the Open Source VirtualBox from SearchServerVirtualization.
Until this year, market share for desktop virtualization products has been divided between two contenders: the various incarnations of VMware – such as the free VMware Player or the $189-a-seat VMware Desktop -- and Microsoft Virtual PC / Virtual Server. Now a new contender has appeared, one which offers a feature mix and a licensing plan that competes very strongly with both of those: VirtualBox.
VirtualBox, now in its 1.4 iteration, exists as both an open-source and commercially-licensed product, with editions available for Windows, OS X (Intel only), and Linux (many distributions), with more potentially on the way. Anyone can use the full commercial version for free as long as they're only using it for evaluation or personal use; corporate users will need to buy licenses.
The open source edition is freely available under the GPL2 license and is functionally the same as the commercial version, although it lacks a few of the more commercial features (mainly, USB and iSCSI support). I worked with the Windows version of VirtualBox for several weeks and was impressed enough with it to start using it as a regular replacement for both VMware and Virtual PC, at least as far as emulating desktop OSes (operating systems) and the occasional server OS as well.
Read the entire review, here.