The Web provides a cornucopia of information about Cocoa (as it does about everything else). We've found it useful to use Google (http://www.google.com, or directly from the Safari web browser's interface) to provide help for the most arcane of issues, including odd compiler error messages. Just type it into the search field and go.
These are the sites that we browse most often for Cocoa information:
Apple uses the Developer Documentation area of this web site to post new documents, and update existing ones, on a frequent basis. In addition, being an ADC member (the basic, online membership is free) gives you access to the latest Developer Tools releases.
Affiliated with O'Reilly & Associates, Inc., the O'Reilly Network is home to the Mac DevCenter, a hub site that offers news, FAQs, original articles, and other technical information for Mac developers and users alike.
MacTech's web site contains a lot of downloadable source code and a monthly column from the print magazine that provides online technologies and resources. These resources include links to web pages, shareware archives, newsgroups, mailing lists, and castanet channels aimed at Macintosh programmers.
One of the original Cocoa sites, Stepwise was created as a resource for NeXTSTEP developers; it serves as an excellent resource for Cocoa and WebObjects programming.
Published on Stepwise, this group of articles written by Bill Cheeseman, serves as a cookbook for developing Mac OS X applications with Cocoa using a no-nonsense, hands-on, step-by-step approach.
This site is updated fairly frequently with tips, tricks, and tutorials for novice Cocoa developers.
This user-editable web site is by and for the Mac OS X developer community. If you've never used the WikiWeb before, this style of site gives literally anyone the capability to view and add information on the site.