Spoedman's GarageBand Stuff
This is a collection of links which are supposed to assist you if you're looking for more, using garageband...
www.voxengo.com Effects - some of which are free
http://www.synthtopia.com/Articles/GaragebandTips2.html
http://www.macidol.com/tips/garageband/ about recording vocals!
http://www.hitsquad.com/smm/freeware/mac/
AU units - specific information
http://www.gersic.com/
http://www.macjams.com/article.php?story=20040329063101758
http://www.thegaragedoor.com/setup/audiounits.html
Mastering
http://blog.macformusicians.com/2010/07/13/mastering-in-garageband/
http://www.youtube.com/watch?v=wkuGJlfTnSw
http://garageband-tips.blogspot.com/2007/12/garageband-tips-mastering-your-songs.html
http://www.thegaragedoor.com/edit/master.html
Links as promoted by http://www.betabugsaudio.com:
(some are very worth your while, so all the credit to these people)