Connectix Virtual PC For Windows

This seems to be a great application e.g. if you have to do software testing on a variety of operating systems and browser versions. It allows you to run Win 3.1, Win95, Win98, WinME, WinNT Workstation, WinNT Server, WinNT Enterprise, Windows 2000 and WinXP on a single machine side-by-side without rebooting.