Continuing from here.
Ch-7: Digging into the source code
In this chapter, Mike describes the tools that work with, around or towards source code: Idlasm, Reflector and FxCop. The book is written for Visual Studio 2003, so some references are slightly outdated - but there is one utility that stands out and is still more useful than ever: Lutz Roeder's Reflector. It is part of the Visual Studio 2005 (albeit in older version that you can get on the Web site).
Here is in a nutshell what does it do for you: it lets you load a compiled assembly (.exe or .dll), shows content of all classes, functions, etc contained in that assembly. It contains built-in disassembler, which is amazingly good. It also contains analyzer, which shows you - at binary level - what functions/methods are called from a given class method or which classes call this method as well as inter-assembly dependencies. Extremly valuable when debugging some low level DLL loading issues.
Reflector has also inspired many add-ins: see the Lutz's blog, or Add-Ins page on CodePlex. The addins do lot of different things: compute and display code metrics, allow to do binary compare of different versions of same assembly, graphically display interdependencies between assemblies, generate unit test stubs ... make your choice.