teahouse. at hitsquad.com is a section on software for the mac.
also try googling.
if you want a nice songwriters aid thats very popular (but beware - highly
addictive). try band in a box from pgmusic.com for the mac sometime. you can try a free demo. basically its centered around loads of song styles. you enter the chords of the song (very easy), and it creates a "back up band" in software playing back. i'm not sure , but i believe the current version lets you add a small number of audio tracks as well. its ideally suited from when i tried it for the person who is composing all by themselves with no access to musicians. it has a huge following amongst songwriters due to its flexibility.
and from talking to many biab users its major strength is to help musicians take a song concept and flesh it out. like a sketchpad as it were.
ive heard some quite amazing stuff done with it. the fact that it creates midi tracks means that you can then import the midi tracks into a full blown piece of multitrack software. then add lots of audio tracks for a finished song.
hope this helps and peace.