You could in theory use Xen or also look at VMware Server and see if that is for you. Xen will likely give you less overhead, but possibly worse performance of guests depending on the type of work as well as if you are able to use the paravirtualization drivers.

VMware server is free now. You may also want to look at Qemu, but VMware will likely solve your problem for you. Figure on losing about <9% of your overall horsepower for the flexibility.