Wikipedia has a great comparison of virtualization products here
I was seriously considering VMWare ESX or Xen or Virtual Iron as the host OS for my main workstation and laptop. Unfortunately, the hardware support is not up to par yet.
Over the past 10 years or so, I have switched OS’s greater times than my underwear! During the last 2 years, I ran Gentoo, MacOS, and Ubuntu as my main OS’s. I will most likely stick with Ubuntu until a solid bare metal vm solution is available with extreme hardware compatibility. At that time, I expect complete bliss. Imagine being able to backup 1 file which will contain a portable vm image along with all of your data within it. No more rsync’ing directories all over the place and backing up configs in case you must reload. I am sick of reloading host OS’s! And hopefully by the time virtualization gets to the level I require, there will be terabyte usb flash drives for easy storage of the vm’s.