One of the tools I had in my toolbox was Reflector. This tool is written by Lutz Roeder and allows you to examine a .NET assembly. Through the use of reflection it can display all namespaces, classes, methods, properties, events and fields in the dll.
It is possible to view the code in IL, C#, VB.NET and Delphi. Some of the useful features are the Call and Callee Graph.
The Call Graph shows you which items are used by a given method, while the Callee Graph displays all the methods that call a given method.
Forgot something for blog readers, the url where to get it: http://www.aisto.com/roeder/dotnet/