VMware might be king of the enterprize market, but I was surprised how far projects like virtualbox have come. It even supports a mixed desktop mode like parallels does on the mac, which is really neat.
http://www.virtualbox.org/
I was trying to use this for a while and I found it to be fairly unstable. At times just minimizing a virtual machine would cause my entire OS to freeze entirely. On my windows box I use the equally free VMware Server.
I actually purchased a license for Parallels due to the mixed desktop. I use it at our RPG sessions to run Excel and OneNote.
I used to find VBox unstable. But as of late, on linux its ultra stable, and quite a bit faster than VMware Server.