C# Call Graph Generation Tool

NDepend is pretty good at this. Additionally Visual Studio 2008 Team System has a bunch of features that allow you to keep track of cyclomatic complexity but its much more basic than NDepend. (Run code analysis)

Concerning NDepend, it can produce some usable call graph like for example (image full size here)

Find more explanations about NDepend call graph here.

It's bit late, but http://sequenceviz.codeplex.com/ is an awesome tool that shows the caller graph/Sequence diagram. The diagrams are generated by reverse engineering .NET Assemblies.