Probably the most popular options for running virtual machines on the Mac are one of two options: Parallel Desktop or VMware Fusion. Both are good working options but neither of them, aside from the trial, are free.   For those of you looking for a free way to experiment with foreign operating systems give Virtualbox a try.  It's an Open Source virtualizer that can handle a large range of OS's including the recently released Windows 7 Beta. Earlier this week Virtualbox was updated and now supports the guest Additions for Windows 7 which make it that much more functional.