Vocal Lab AU is a plug-in that helps you train to sing on key. It listens while you sing and displays the pitch of your voice on a graph, in real time. All you have to do is watch the graph while you sing and try to stay near the lines. With a little practice you can greatly improve your intonation.
It can identify problem areas in your singing, and can also help improve your intonation on various instruments - Violin, Brass, Woodwinds, and more.