There are many vendors who develop such tools. Below are two leaders in this domain that i know:
1. NDoc (Recommended my Microsoft) http://ndoc.sourceforge.net/ - NDoc generates class library documentation from .NET assemblies and the XML documentation files generated by the C# compiler
- NDoc uses pluggable documenters to generate documentation in several different formats, including the MSDN-style HTML Help format (.chm), the Visual Studio .NET Help format (HTML Help 2), and MSDN-online style web pages
- NDoc's source code is freely available
The ASP.NET Documentation Tool creates technical documentation for ASP.NET 1.0, 1.1 and 2.0 projects written in VB.NET and C#. It is intended to the quickest and most cost effective way of automatically documenting and understanding ASP.NET projects. It is also able to document Windows Forms and Console applications written in .NET.
Documentation is created in standard HTML, Microsoft's HTML Help and plain text formats.
One of primary input for such tools are comments within the source code
FYI - Documents created by such tools has a high level picture about the source code i.e. namespaces, classes, Functions, subroutines, web controls, any other component, etc that are used in the source code. This is completely for developers which can be useful for future reference and can also be helpful for new team members. Please refer to above URLs for more details about the product.