This is kinda cool, and simple to make too. Basically the insides of an optical mouse put inside halves of clear bottles and wrapped in fabric. The result is a Soap Mouse, named after the way you control it.
Check it out on hacked gadgets:
http://hackedgadgets.com/2006/09/28/soap-mouse/
Youtube Video:
http://www.youtube.com/watch?v=hohu8SSpduM