SandCastle is a documentation compiler for managed class libraries, that will output your comments/documentation with a MSDN "look and feel".
Here's the link to the March CTP: http://www.microsoft.com/downloads/details.aspx?familyid=E82EA71D-DA89-42EE-A715-696E3A4873B2&mg_id=10111&displaylang=en
You can read the technical details on its architecture and approach from this link on the SandCastle blog site: http://blogs.msdn.com/sandcastle/archive/2006/12/06/a-microsoft-code-name-a-day-sandcastle.aspx
I really like one of the add-on GUI's that you can use on top of sandcastle, called SandcastleGUI. This free addition to the sandcastle CTP allows you to do a lot of things, but one of the most interesting is that you can add video examples to you documentation. With that, you can actually show developers how/why to use what you've built, which goes beyond basic documentation. Dust of you webcams and practice your most "cheesy" professional speakers voice (think your local weather man. You know the guy with the 1970's sports coat and the fake smile), that's who you want to imitate. :)
Here's the link to sandcastleUI: http://www.inchl.nl/SandcastleGUI/
~ Robert Shelton