This is a good app to run multiple/older versions of Windows under Windows (like in Boot Camp). Makes it nice if you're switching and want to dump your old development PCs. If you want to do this inside OS X, you can run Parallels, but if you happen to work in XP under Boot Camp often, you may find this useful (think of it as Parallels but for Windows). This is NOT the Mac version of VPC, unfortunately.
http://www.microsoft.com/windows/virtualpc/default.mspx
http://www.microsoft.com/windows/virtualpc/default.mspx