It doesn't matter if you're running on Windows or Mac OS X - every power user needs either Parallels or VMware (or both). There's never been an easier way to test software without destroying your primary operating system's stability. Think of how many times you wish you could press a 'reverse' button on your computer. Plus, there's no easier way to try new Linux distributions - see what all the fuss is about. Run Windows in OS X, run Linux in Windows, but the best way to do either is with VMware and/or Parallels.