I used to work in a Java shop where we had all kinds of documentation. We had a full-on API. It was very useful, and even in the cases where it needed a little updating, it helped you figure out where to begin looking for whatever you were looking for. I remember there was some fancy thing in Eclipse that would even create the javadoc code for you if you were willing to add a new method via some contrived WYSIWYG method. I've never worked with anything like that in .NET, but that's ok, it's pretty easy to work backward in the latest version of Visual Studio.
Looking for others..