Do you mean something that will allow you to write scores or something that will allow you to write music via "virtual instrument" based on setting it up over a track? Garageband has the instruments and stuff. that will allow you to do this. If you want to do pattern based writing, like beats, loops and similar tracks, fruity loops will allow you to do that as well. Logic and DP will allow you to do midi tracks as well, but are more expensive. There is logic express which is fairly cheap.
If you mean something like a score with sheet music, I'm not exactly sure.