Photo credit: cynwulf

I'm of the opinion that in-line comments are often a "code smell", but if you're developing a framework or class library that you want others to use, .NET XML comments on public classes and members used to generate documentation can be very useful.

The tool for building that documentation is Sandcastle. According to its CodePlex site, "Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source assemblies and optionally integrating XML Documentation Comments."

It's been a long, long time since Sandcastle was updated (2+ years), but version 2.6 was finally released June 23rd: http://sandcastle.codeplex.com/releases/view/47665

Another open source tool, the Sandcastle Help File Builder, is a GUI tool that makes working with Sandcastle much easier:

The latest version of the Sandcastle Help File Builder was released July 7th, supporting the Sandcastle June 2010 Release: http://shfb.codeplex.com/releases/view/40105

For creating the XML comments in your code that will be used by Sandcastle to build documentation, I highly recommend GhostDoc: http://submain.com/products/ghostdoc.aspx

GhostDoc is a free Visual Studio extension that automatically generates XML documentation comments for methods and properties based on their type, parameters, name, and other contextual information.