The latest in customization is called Keyboard Sounder, a program that assigns each key a unique sound. The mouse can also be customized with adding sounds to the clicks.
The sounds are organized by theme, which include everything from a classic typewriter to gunshots or punches. There are six themes included by default, although you can download more sound packs from the program's website.
The total amount of sounds in one theme is at most 18. That way the letter keys all maintain the same sound, which doesn't happen with special keys like the space bar, return, home, delete, enter, etc.