Reflector is the class browser, explorer, analyzer and documentation viewer for .NET. Reflector allows to easily view, navigate, search, decompile and analyze .NET assemblies in C#, Visual Basic and IL.

Download it here, and check out the add-ins available here.

I've used it extensively in the past, and while I'm not quite as ga-ga over it as Scott Hanselman, it's definitely a worthwhile tool for your toolbox.