[msbarnett]

Mike Ash's Friday Q&A series is always a great investigation of the depths of Objective-C and Cocoa: http://www.mikeash.com/pyblog/

[eelco]

For iOS/Mac development, Ole Begemann's http://oleb.net/ is invaluable, especially for the monthly link roundups.

[msbarnett]

Awesome, I wasn't aware of that one.

Other good ones I know of are the already mentioned Cocoa with Love

http://cocoawithlove.com/

Cocoa is my girlfriend

http://www.cimgf.com/

and bbum's weblog-o-matic

http://www.friday.com/bbum/