Various ideas:

• scrabble/puzzles/pictionary

• Gambling (poker, blackjack, roulette, etc)

• projection-guided manipulation (eg: picking bad apples out of a stream)

• assembling things

• sharing applications

• entertainment

• finger-length based biometrics, user identification

Puzzle assembling:

1. A number of jigsaw puzzle pieces are scattered on the table, face up

2. as I put my finger on any puzzle piece, all the other ones which can snap with the chosen piece are illuminated.