Virtualbox [free]
[linux or windows] host and [linux or windows] clients
file type: vdi
1 processor per client
can convert VMWare images to vdi
can use dd images
spawns one process on the host per client
VMWare
[linux or windows] host and [linux or windows] clients
limited to 2 processors for each client
ESX is limited to 4 processors per client
VMWare converter: takes a live Windows system and produces a vmware image
XenExpress
http://www.howtoforge.com/virtualization_with_xenexpress
Free
limited to 4GB RAM, 4 virtual machines
VirtualPC
Windows host and windows clients only