Synthesia

Synthesia is a free game that teaches you to play the piano using falling notes. With it, you can: Learn to play songs using a musical keyboard connected to your computer. Keep track of your progress in every song with per-song scoreboards. Slow, fast-forward, and rewind songs while learning new parts. Practice left and right hand parts together or separately. Try out any MIDI file you can find on the Internet. With the new (completely optional) for-pay "Learning Pack" component there are a couple more features you can unlock. Have an easier time learning the notes in a song with the new practice mode. And, view songs as sheet music in addition to the falling note display.