.NET Reflector is seriously cool. At first glance, it's like ildasm, letting you explore assemblies and see the namespaces, classes, functions, etc. and see the disassembly of the code. But, it goes much further. One really cool feature is the ability to decompile the routines back into C#. Obviously it doesn't have enough information to give you the exact original code, but it does a surprisingly good job of generating readable code! It also lets you see every function a function calls, and every function that calls a function. And, it has some nice searching features (replacing the need for Code Viewer). It's a great way to explore a codebase. Version 4.0 was just released.
On MacOS X, you can use class-dump to dump the info from a Cocoa app/framework. I'm surprised there aren't visual tools for dealing with this data. I found Class-Dump GUI, but the installer seems to be missing the actual app or something. Maybe there's something out there I don't know about.