Performous is an open source karaoke/instruments game, in the same vein as Rock Band. Perfect for practicing your favorite Jonathan Coulton song!
While Performous might be classified as a karaoke program, it is actually much more than that. Instead of just displaying the lyrics, notes are also displayed and the performance is scored based on how well you actually hit the notes. Unlike in most other games in this genre, you will also see the precise pitch that you are singing, so that you can see what you are doing wrong and easily (well, everything is relative) correct your pitch.