VMware
From Burden's Landing
VMware Workstation
Costs about $180.
VMware ESXi
A barebones hypervisor. It does not require a host OS -- it is a host OS, which in theory means it does a better job of managing resources. It also means in practice that it's hardware support is less extensive. For enterprise servers, it's not an issue, but for desktop use, it doesn't make sense.
Free, so the price is right.
