VMWare for way less than 10K a pop.
If you're running a multi-million dollar/pound datacenter, 10K a pop is not that big a deal. If you are an SMB or running a test lab you can download and install ESXi and run it for free. The free license gives you the ability to run ESXi on some number (10? 50?) of servers indefinitely. Unlike the free VMWare Server product, with ESXi you can create and maintain multiple snapshots of your various guests, configure VLANs and such on the fancy included soft switch, pass through up to four CPU cores to a guest, connect multiple ESXi servers to your SAN using iSCSI, etc. I have four of them in my lab right now, hosting from one to four guests each. It's a brilliant solution for testing. The ESXi hypervisor installs and configures in all of roughly five or six minutes. I'm using 4.1 and it is a nice improvement over 4.0 in terms of reduced network latency and jitter on the guests. You can run a lot of less disk intensive guests straight off a low-end SAN if you wish (Openfiler, anyone?).
What you don't get with the free ESXi is the ability to run VCenter (I think that's what its called) and manage all of your ESX(i) servers simultaneously with one tool, move running guests seamlessly from one host to another, etc.