A CTP version of Microsoft's Sandcastle was just released.
What is Sandcastle's purpose?
“To enable managed class library developers throughout the world to easily create accurate, informative documentation with a common look and feel.”
Read more about it here -> http://blogs.msdn.com/sandcastle/
Download it here - > http://www.microsoft.com/downloads/details.aspx?FamilyId=E82EA71D-DA89-42EE-A715-696E3A4873B2&displaylang=en