MaKey MaKey has a awesome little kit for thinkers and dreamers for sale by 2 graduate students at MIT Media Lab. You can use a USB cord that connects to a circuit board that you can connect to your computer. You can work the circuit board to take over functionality of certain keys or functions.
For example you can use almost anything as your “buttons” to perform actions on your computer. Thinkgeek’s store suggests to use alligator clips and a banana (pencil or play-doh what have you) to make the functions workable in creative ways. Basically as long as the object can conduct electricity, you can use it as part of your controller. It works on all 3 platforms of Windows, OS X, and Linux, and you can even combine multiple kits to make your inventing more fun! You can order it here through Thinkgeek’s online store! A top recommendation for a unique gift!