Roland Weigelt's GhostDoc is a Visual Studio add-in that automatically generates C# XML comments. By examining property, method and parameter names it generates a comment block that contains the core comment elements. I find it particularly useful in correcting comment blocks when the parameter list of a method changes.

Because it generates the comment from an examination of the code it doesn't add information that cannot be inferred from the code. Therefore in almost all cases the comment block still needs to be edited to include more descriptive information. The advantage of using GhostDoc is that it does the obvious, tedious bits for you. This can be accomplished with a single keyboard shortcut. Occassionally the text analysis fails and produces a result that makes no sense. I'm particularly fond of the "Mains the specified args" it produces for the Main method of console apps. On the whole though the text analysis is fairly good and the comments produced require mostly extension rather than editing.

GhostDoc is a good example of a small, focused add-in that does one thing and does it well. It's free (but not open source, check the license) and very solid. Just remember, if you ever need to uninstall Visual Studio uninstall GhostDoc first or you'll need to edit the registry to remove it.