https://github.com/marek-stoj/ImmDoc.NET
ImmDoc .NET is a command-line utility for generating HTML documentation from a set of .NET assemblies and XML files created by the compiler. It's developed in C#.
See an example of documentation generated by ImmDoc.NET.
The distinguishing feature of ImmDoc.NET is that unlike Sandcastle or NDoc it doesn't require you to have all referenced assemblies. That is, if you have a class C1 in assembly A and a class C2 in assembly B and the class C1 has a field of type C2, then you don't need to have the DLL file with the assembly B to generate documentation.
You can also read an article about ImmDoc.NET at CodeProject (note however that it may be a little outdated).