One thin container (transparent plastic bag) full of dark coloured liquid, one webcam and a glass table and you get your own multitouch interface. Ingenious and very simple. Tom White seems to have come up with this for his Master’s Thesis at MIT in 1998 (check out the comments on the page linked to by the title).
Here’s a video of someone who — seems to have independently thought of the same idea and — actually implemented it on their Mac, using his iSight and writing the necessary control software in Cocoa.
