CocoaBuilder, Searchable Cocoa mailing list archives
Stepwise, includes news items of interest to Cocoa developers. The maintainers of this site have two books out (Cocoa Programming, and Vermont Recipes) Also has a set of top headlines of interest to Developers.
Cocoa Dev Central. A nice collection of tutorials for new Cocoa programmers.
HyperJeff's Cocoa Literature List, searchable by title and by content.
Stanford's CS193E, Object Oriented User Interface programming, using the Hillegass Cocoa book.
What is Mac OS X?, architecture, boot process, features, etc.
Cocoa Programming by Erik M. Buck, Donald A. Yacktman, and Scott Anguish. This is a must-have for experienced Cocoa programmers. If you're just starting out, get Aaron's book first.
Cocoa Recipes for Mac OS X by Bill Cheeseman. This is a step-by-step book that builds a Document-based application, and then evolves it, adding features to the program and data to the document. It shows one way to structure larger, more complex applications. The writing can be a little dry and tedious at times (the author is a retired lawyer) but there is a lot of good information in here, including some stuff not covered anywhere else (such as the help system)
Core Mac OS X and Unix Programming, by CocoaHeads co-founder Mark Dalrymple, and Aaron Hillegass. It covers primarily unix programming, but there's a fair amount of Cocoa in there, including NSTask, Distributed Objects, plus coverage of performance issues.
Learning Cocoa with Objective-C, by James Duncan Davidson. This is the second version of O'Reilly's introduction book.
Programming in Objective-C by Stephen Kochan. This is a tutorial on Objective-C, without assuming any prior experience with plain-old C.
Objective C Pocket Reference by Andrew M. Duncan. An O'Reilly pocket guide that distills the most importation information you need to know about Objective-C.