Multi-touch interface for the mac built using a bag of water, dye and a camera. There is brief discussion of how he did it on the blog linked below, but should be easy to figure it out.
http://blog.medallia.com/2007/06/dyesight.html

People seem to be having issue with the $2 claim, but if you have the stuff lying around and don’t have to code it ... pretty cheap.
Someone dugg, if you dig it, um dig it here: http://digg.com/apple/dyeSight_the_2_Multi_Touch_Pad

Thanks and welcome corntoole. Someone started a post yesterday regarding the same thing. You can check it out here: http://nuigroup.com/forums/viewthread/339/

sure if you have the stuff laying around, but that doesnt mean that stuff you have laying around is for free

Just a quick pointer: you could only use this as an interface, you wouldn’t be able to project an image onto it.  But its still pretty cool, and it would be a good test for your programs.