The stuff at CocoaDevCentral is really good though sometimes they try to make things out to be simpler than they are (see their recent Cocoa Bindings and Core Data demos which try to convince you you don't have to write any code at all).
A couple of days ago I downloaded Apple's 'CoreRecipes' sample code (it's a Cocoa core data demo) and the full app included there is a lovely bit of advanced code. There's so many good ideas in it - I learnt a lot just reading the code. It's mostly Core Data based obviously but the best bits for me are the implementations of 'Smart Folders', the 'Predicate Editor' UI code and custom NSViews. There are also implementations of a Automator action and a Spotlight importer. The coding style is very clean as well.
It's definately not for beginners but I'd recommend anyone with a bit of experience to read it.