Audio+Plugins

Audio Plugins allow you to add additional effects and capabilities to Garageband

http://techuniversity.com/items/garageband-chiptunes?utm_source=theappleblog&utm_medium=editorial&utm_term=screencast&utm_campaign=promopost_free

Magical 8bit Plug is a plug-in style software-synthesizer for producing primitive electronic sounds like the old 8bit game consoles. In Macintosh environment, it runs with AU (Audio Units)-supporting applications: ex. Logic, Digital Performer, Garageband. http://ymck.net/english/download/index.html