What are some unusual uses of the MacBook trackpad?
In the "unusual" camp there's a program called Inklet that purports to turn your trackpad into a Wacom-like tablet. They also sell a special stylus and sketching software.

For music there's Trackpad Magic, which allows you to create music using trackpad for input, and djay, a DJ'ing app that lets you "scratch" records with your trackpad.