Fill Linux to the Brim with Xen
Linux + Xen = Mighty Virtualizing
Linux is a superior platform for supporting 64-bit processors, multi-processors, and blade/cluster architectures, plus you don't get trapped in licensing hell, so it's a natural as a host system for all sorts of virtualization scenarios. You get horsepower, stability and flexibility. Linux + Xen promises to be the real VMWare killer, for those who require "killer" scenarios. (Us peaceniks will settle for "something that works faster and more better. Oh yeah, and doesn't have a big price tag.")
Xen is just a baby, though a fast-growing baby, so it's still pretty limited. See the OS compatibility page for current status. Xen supports up to 4GB RAM, and currently supports only IA32, so it won't run on Itanium. It will run on any AMD64, since these have native support for IA32. Xen supports SMP on the host system, but not the guest OSes.