DoubleCommand

DoubleCommand is software for Mac OS X (a kernel extension) that lets you remap keys, in other words change the way your keyboard works. Often used to make a PC keyboard more comfortable with a Mac, swapping the Alt (Option) and Windows (Command or Apple) keys, since they are in swapped positions on Mac and PC keyboards. DoubleCommand is a quick and easy way to fix this if you use a PC keyboard with your Mac.

Get it here, or download here if the site is down..

Get it here if you want Mavericks support (or here).