I second @aku for GhostDoc. I use it every single day on the purchased code-base that I maintain and extend. I've bound ctrl-shift-d, d to generate documentation and ctrl-shift-d, r to regenerate documentation.
Wonderful, just wonderful.
Don't expect the generated documentation to be extraordinarily accurate and helpful, however. For
public string DocumentGenerate(int generationParameter)
It will generate something like
/// <summary>
/// Generates the Document
/// </summary
/// <param "generationParameter">The generation Parameter</param>
/// <returns></returns>
Which, while accurate, could be more helpful. You'll need to flesh out the documentation.