Synthesia is a brilliant little game for Windows that works much like Guitar Hero, but for the piano. It's fully customizable, and can be used just for fun, or as a real training tool.
Select a song and Synthesia will highlight each key you need to hit in order to play it. You can play using your ordinary PC keyboard, or attach a MIDI keyboard if you're interested in learning to play properly (or you have one gathering dust in the garage).
Why you need it
Synthesia lets you play with one hand or both, and there are specific options for practicing the melody (with the song pausing until you hit the right note), the rhythm (with the song moving on at a constant speed), or a full recital (one playthrough at full speed). At the end of each attempt you're awarded points for your performance.
The free edition of Synthesia comes with a set of songs of varying difficulty levels, and you can buy a key to unlock more if you want more of a challenge.
Download here: Synthesia