ReKey for Mac

From Infinity-to-the-Power-of-Infinity:

ReKey uses one of the many interesting features of Mac OS X's Cocoa to allow you to change the keyboard shortcuts in any Cocoa application, without actually modifying the application itself. All your modifications are saved in the preferences file of the application, and therefore do not affect other users of the system.

Because of limitations in the Carbon Application Programming Interface, you can only modify the shortcuts of Cocoa applications. Cocoa applications are Mac OS X applications that only run on Mac OS X, and do not have Mac OS 9 compatibility.