Rich GUI OS X Frameworks?

Cocoa. Considered by many to be the best application framework ever. The language is Objective-C, SmallTalk-like language that inspired the creators of Java.

Really, there is no reasonable alternative to Cocoa for OS X development, unless you have specific needs like wanting to be cross-platform.

Aside from Interface Builder which is included as part of the Xcode tools, you can also use QT, GTK+, AWT & SWING (for your Java background), Tk, Squeak (for your Smalltalk background), Shoes (very cool little Ruby GUI toolkit), FXRuby (more Ruby), wxWidgets, XULRunner, and others I'm sure I've forgotten. For the most native-like apps, however, Interface Builder is your best bet.

Cocoa is the primary framework to use on Mac OS X. It's what Apple uses, it's what most new development uses, and it's where new features are principally added.

If you're coming from WPF, I think you might find quite a few of the concepts in Cocoa familiar. (Despite the fact that Cocoa is just a bit older.) It's built entirely around MVC, there are property-change notifications and bindings, there's animation support, there's a persistence and object-graph management framework, and so on.

(Also, you might want to add "mac" to the tags.)