http://www.codebeach.org/

CodeBeach aims to be a central repository where Mac developers can share useful code classes and snippets. Submitted code is considered to be in the public domain.

CodeBeach is provided by briksoftware, who have contributed a number of classes to start things off.

http://homepage.mac.com/mmalc/CocoaExamples/controllers.html

A collection of examples that illustrate different aspects of the controller technology within Cocoa Bindings.

http://boredzo.org/opensourcelinks/

A collection of links to reusable source code available under free licenses.

http://developer.apple.com/samplecode/Cocoa/index.html

Along with the code examples that get installed with the development tools, the sample code on ADC provides a comprehensive selection of examples to get you started, and to help you explore more complex areas of Cocoa development.

http://osx.hyperjeff.net/Apps/

The best-kept list of OS X applications, listing an amazing 17,000+ applications, widgets, frameworks, libraries and more. Can be filtered by license type and whether source is available.

http://www.dejal.com/developer/

A collection of useful categories to extend AppKit and Foundation, compatible with Mac OS X 10.3 and later.

http://www.cocoatraces.com

Cocoa Traces has been designed as a central repository for useful pieces of Objective-C Cocoa source code. The site organizes fragments of source code, based on different topics, into traces which can be searched.

Cocoa Traces is provided by intuiware.com, who have contributed a few traces to start things off.