gandbug

gandbug is a runtime debugging system for your Cocoa applications. Once added to your application code base (via a simple static library and one line of code), it will add a single menu which will use a variety of introspection techniques to help you understand how your application is really structured and how it really is working.
It will show you (besides other things):

The visual heirachy of your windows - see how things are really layed out, and what the responder chain is.
Find all bundles and frameworks currently loaded in your application.
Dump the contents of the pasteboard to see if it contains what you think it does.
Listen to all the notifications, both in your applications, as well as distributed notifications.
Show the controller bindings used - what views are bound to what parts of your model, and eavesdrop on the notification mechanism to see them in real time - while you are using the UI.
Examine all the defaults, both of your application as well as system defaults.
Find out what dyld has loaded into your application, and where.

What's more, all the objects displayed don't just log a simple description - they will display:

Contents of container objects - whats in an array, dictionary, and will dump the memory of a data object.
See all the instance variables, as well as retain count (which can be further examined in detail)
Execute any accessor method, and examine the results of that.
Go up the class heirarchy to see the instance variables and accessor methods that are inherited from superclasses.

All of this is sent to a stand-alone application (which helps to minimize the impact on your UI, as well as survives your application crashing). It is presented in an outline form, which allows you to progressively see more detail (or hide them). This log can also have unneeded details removed, comments added, and then copied to the pasteboard or exported to a text file.
As of version 1.0b4, the following major features have been added:

A graphing interface to show object relationships or view heirarchies
An interactive expression evaluator which will, besides telling you what 2 + 2 is, allow you to send messages to your objects interactively, so you can manipulate your application from the "inside".
The ability to trace arbitrary method execution - marvel as an NSScrollView gets sent a tile message while resizing.