I've already used NDoc (http://ndoc.sourceforge.net) for a few years, so I'm always eager to fill in the comments properly. But it is quite annoying that I have to type the documentation of an override again and again. GhostDoc adds a context-sensitive menu entry and a shortcut-key, and uses a intelligent configurable rule dictionairy to generate the comments from the definition, base-class member, etc. It even copies the documentation from the MSDN.
Check it out at http://www.roland-weigelt.de/ghostdoc.