VMWare is now offering ESX for free! I have been using it on my test system and it works great. I can now run multiple versions of Notification Server and Deployment Server on separate virtual machines.
For me, the best part about ESX is that all the guest OSs share common memory pages. That means you can have four Windows Server 2003 virtual machines up and since most of the data stored in memory is the same, they cumulatively will use about the same RAM as two servers.