Apple Developer Connection:Provides news and technical information for Apple Developers.
GNUstep Project: A project devoted to providing an object oriented application framework to developers.
iDevGames: Macintosh game development website featuring news, tutorials, game assets, forums and articles.
Mac OS 8 and 9 Developer Documentation: Gateway to Apple's documentation for programming Mac OS 8 and 9, including Carbon development.
Macdev at MIT: Various resources for Macintosh development from the folks over at MIT.
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!
MacOS Web Directory: A directory of Mac programming related sites, including development kits, format specs, toolkits, open source, tutorials, and webzines.
Sound & Vision, Inc.: Develop software for Macintosh platform. On this site users can find information about FineReader 5 Pro, development and layout software.