One of the most complete and up to date products comparison (feature by feature):

Virtual Infrastructure products: features comparison - http://www.it20.info/misc/virtualizationscomparison.htm

 

VMware products comparison:

http://deinoscloud.wordpress.com/2009/10/21/vmware-esx-and-esxi-4-0-comparison/

ESXi vs Full ESX

VI 3.5 vs vSphere 4.0

VMware Server vs VMware ESX/ESXi