XenServer
"XenServer is free and so does not drive much revenue on its own at Citrix – there is some support stream for customers who use it as their server virtualization layer"
$5000/server doesn't seem like free to me:
http://www.citrix.com/English/ps2/products/subfeature.asp?contentID=2300456
Sure, there IS a free version, but you don't get all of the features. There's also XCP (the OSS spinoff), but you don't get support for that at all!