Free Loops
Free loops (no sign-up required) http://freegaragebandloops.com/
Some cool free loops: http://www.looperman.com/loops.php
Free loops (need to sign up for free membership): http://www.macloops.com/
Large look library at http://www.samples4.com/catalog/index.php?ref=11&affiliate_banner_id=2 linked from http://www.thegaragedoor.com/loops/open.html
http://soundbible.com/
Free Synth/Effects Plug-ins
Some commercial and free plug-ins: http://www.kvraudio.com/plugin-ranks.php
Look, 8-bit Nintendo sounds as plug-ins!: http://createdigitalmusic.com/2006/07/10/8-bit-nintendo-sounds-as-a-free-windowsmac-plug-in-add-an-nes-to-garageband-stompbox-nes-fx/
Free Audio Unit Plug-ins: http://www.synthtopia.com/content/2007/04/22/free-au-plugins/
Direct download for Plug-ins: http://mda.smartelectronix.com/effects.htm
http://www.thegaragedoor.com/gear/plugins.html includes some useful links
http://www.4trackmag.com/2010/03/free-synthesizer-plugin-for-garageband-or-logic/
