VMware		Microsoft		Sun
VMware ESXI 4 64 Bit		Hyper V Server 2008 R2 64 Bit		VirtualBox 32 & 64 Bit
VMware ESXI 3.5 32 Bit		Virtual PC 2007 32 Bit		OpenLink
VMware Player 32 & 64 Bit		Windows XP Mode and Windows Virtual PC for Windows 7 32 & 64 Bit		Virtuoso Open Source
VMware Server 32 & 64 Bit		Virtual Server 2005 R2 32 & 64 Bit		Many Other VM Products here

Virtualization is still evolving. Microsoft released Hyper-V 2008 32 and 64 bit and was quickly replaced by Hyper-V 2008 R2 64 bit only. It is almost impossible to find the download for the first 2008 versions. VMware, the Enterprise VM industry leader, is adding more support products, changing the names and versions at a rapid pace, adding confusion to their product line. For development, both are good products, however the products need to mature and have more security in place. Using these products in live production, expecting a 5 year stable platform, without a lot of maintenance is definitely not a reality at this time.  Parallels Virtuozzo  is preferred by the major virtual hosting providers. Computer virtualization is nearly half a century old and the race is on to see which company will dominate the "new" revival, especially with Cloud Computing.

Parallels

VMware

Microsoft

Novell

The difference between  OS virtualization and hardware hypervisor virtualization Flash Video