Batukan Bozkurt's Touch Pianist is a single-serving site where you play the piano. The magic: you don't have to worry about the notes, only the rythym. It's marvelous fun, and surprisingly enlightening, to tap your way through the Moonlight Sonata or any of the other classics in the system–especially when you apply a bizarre time signature where it absolutely does not belong.
Touch Pianist is a musical toy / instrument that allows the user to perform hard-to-play classical piano music favorites (from composers like Beethoven, Mozart, Bach, Satie, Debussy to name a few) just by tapping the rhythm of the piece's particular sound events on a computer keyboard or a touch screen. It also provides live visualisation of the pieces that are being performed. There is no score keeping, the user is free to perform the music in any way they wish. The premise is that it is really fun to perform a piece you know well, exactly like you feel it with so little effort.
Embedded further down is a video of the app version of it in action.