This is an application that works through the System Preferences panel which allows the hacking and remapping of specific keys, i.e. creating a numpad function on an 87 key keyboard. This will allow you to also assign key functions like eject, media controls, as well as others to the keys of your choice. To top things off it even has certain functions for specific applications such as terminal, firefox, starcraft, and other applications, and its free! I have found a few applications that sort of do the same thing, but this is BY FAR the best one I have come across. Hope this helps.
http://pqrs.org/macosx/keyremap4macbook/index.html
