is a well-regarded
keyboard macro application for OS X that lets you define and use a variety of custom keyboard shortcuts. Macros are basically scripts that can be used to perform a variety of actions, including manipulating on-screen windows, simulating mouse and keyboard buttons, controlling the clipboard, etc. Version 2.1 is Universal and includes several bug fixes.
If you're interested in automating certain tasks on your Mac, or even over-riding keyboard shortcuts in certain applications, Keyboard Maestro's macros might very well do the trick.
Keyboard Maestro is $20US and a demo