VMWare Fusion. (for Macs)
http://www.vmware.com/products/fusion/
I have heard that name floating around a lot in the last few years. I now understand why. I use it at work for our servers, and on my desktop. Installs right on top of you OS, just like any other app. Allows you to run ANY other os without major changes to boot records, formatting, file structure ect. Its great, and easy to use.