ESXi- VMWare's lightest and easiest Hypervisor

ESXi is lightweight and has almost all the features of ESX.  It is super easy to setup.

But did you know you can run it or install it from a USB key?  Well not at the same time, but check this out:

http://blog.torh.net/2009/02/22/running-vmware-esxi-hypervisor/

http://www.squishnet.com/?p=17

They are a little windows centric.  If you are on OS X or Linux it is easier (as you have dd and the linux boot tools on there.  But you get the point.