Whoa! It is VERY interesting! Maybe it can make even easier to make your DIY arcade cabinet?
Also, obviously you can make your own DDR dance pad (not with water though... I would use metal plates, make more sense).
Or create some wacky musical instruments...
Too bad it only interfaces with keyboard it seems, thus lacking some serious analog input... Although I don't know if it CAN track analog input anyway.
The project started about a year ago on Kickstarter and raised $568K http://www.kickstarter.com/projects/joylabs/makey-makey-an-i...
It's a lot of fun to play with and a great educational experience. They usually have a fun booth at Maker Faire.