"Parallels Desktop for Mac enables you to run Windows, Linux, and more side-by-side with Mac OS X on any Intel-powered Mac, without rebooting. With Parallels’ award-winning virtualization technology, you can run Mac OS X and your favorite Windows applications at the same time. You can even drag-and-drop files between desktops!"
Very nice to be able to run windows when needed, with a click of the mouse. Haven't got Half Life 2 working, yet. What's up with that?