Microsoft has recently released Virtual Server 2005 R2 available as download for free. (full product, not evaluation) Vmware has also released free beta software for their upcoming free Server virtualization, (aimed to replace their current GSX Server) Also full lisenced software, not evaluation.
VMware Server beta download
Microsoft Virtual Server 2005 R2 Full Download
if you ask my opinion, Vmware will probably win the war just because they've been in the business longer than MS, but you never know i guess, I've just always liked it more and always seemed to get more publicity than virtual PC or VMware Workstation.