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.