In my experience, VMWare pwns Virtualbox and Virtual PC. Then again, VMWare isn't free.
Both VMWare Server 1.09 and Server 2.0 are free. The major difference between Server and Workstation (along with the paid versions) are tech support, Workstation gives DirectX 9.0c support for the guest OS'es, as well as Workstation has drag and drop file transferring from host and guest. Workstation also has better usb and sound support.
I've used both VMWare Server 1.09 and 2.0 without a problem and seem to be more feature filled than VirtualBox. To get Server 1.09 and Server 2.0 all you have you do is register for a cd-key and then download the .exe, .rpm, or .tar.gz depending on your host OS.