I forgot to mention VMware player. The only problem with it is the fact that you can only use images that are already created (there is a hack, but its a lot of work).
Why can't they make VMware Workstation free! That would make a lot more since to me and make things a lot more legal for people needing virtual machines.
Story via CNET News
There are free virtual machine programs out there; Bochs, QEMU (very slow when I used it), Xen but none of them compare to the capabilities of VMware Workstation. I prefer it over Microsoft's Virtual PC.