grack.com

Fabrice Bellard has recently added kernel monitor support to QEMU (The QEMU Accelerator Module, or kqemu), allowing it to execute simple instructions natively on the CPU - much like other virtualization tools like VMware do. 

The speed improvement is dramatic and places QEMU near the the top with other virtualization technologies, though it lacks the high-quality GUI and administration tools of the other enterprise products.  Best of all, it’s completely free!