Development

GNUstep Project
A project devoted to providing an object oriented application framework to developers.
http://www.gnustep.org/

iDevGames
Macintosh game development website featuring news, tutorials, game assets, forums and articles.
http://www.idevgames.com/

Mac OS 8 and 9 Developer Documentation
Gateway to Apple's documentation for programming Mac OS 8 and 9, including Carbon development.
http://developer.apple.com/macos/

Macdev at MIT
Various resources for Macintosh development from the folks over at MIT.
http://web.mit.edu/macdev/

MacHack
"The Annual Conference for Leading Edge Developers"; a multi-day round-the-clock ritual involving gallons of soda, heinous hacks, and lots of camaraderie. Not to be missed!
http://www.machack.com/

MacOS Web Directory
A directory of Mac programming related sites, including development kits, format specs, toolkits, open source, tutorials, and webzines.
http://www.strout.net/macweb.cgi?dir=/Programming