In the virtualization space, everyone follows VM Ware. No one else is even close in mind or market share. But the market is going to expand very rapidly and Xen, which is free software, hopes to become a major player. They have the advantage of very low cost (free) and very favorable press reception.
There are some nice points made in this article. If you have any interest at all in virtualization technologies and where they are headed, this one is for you.