First start here: http://www.opensourcemac.org/

And here: http://www.ajaxflakes.com/open-source/top-100-best-open-source-mac-software/ 

And finally here: http://www.apple.com/downloads/macosx/unix_open_source/ 

More to follow here.

http://www.macdevcenter.com/ 

http://www.freesmug.org/ 

http://www.cocoaforge.com/ 

http://www.macosforge.org/ 

http://www.macforge.net/