Sandcastle - Create MSDN-style documentation for your .Net assemblies
I see that a CTP of Sandcastle has just been launched. From their blog:
"Enabling managed class library developers throughout the world to easily create accurate, informative documentation with a common look and feel"
Back when I was building enterprise apps, I used to be a huge fan of nDoc, but sadly that project withered on the vine around the release of Whidbey. Now once again we've got a tool to quickly spin up reference documentation on our assemblies. Nice.