http://www.csounds.com/catalog/ Richard Boulanger's CD ROM Csound catalog.
http://www.babcom.u-net.com/csound.html A large collection of instruments by Steven Cook.
http://www.geocities.com/SiliconValley/Bit/5683/ Varga István is an excellent sound designer and his page has lots of great code.
http://www.haint.com/sounds/ Sean Costello has some excellent sounds available on his web site.
http://www.dspdimension.com/ Stephen M. Sprenger'sAudio DSP Pages. An excellent collection of audio DSP articles by one of the leaders in the field.
The Usual Links
The Csound Front Page http://mitpress.mit.edu/e-books/csound/frontpage.html