Krzysztof Cwalina and the Framework Design Guidelines team have come up with a tool called “Framework Design Studio”. This tool helps the API designers design APIs for their frameworks like .net Base Class Library.
For long Cwalina and Brad Abrahams has created a guidelines on how to design API which now is available as book – Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries (http://www.amazon.com/exec/obidos/ASIN/0321246756/bradabramsblo-20)
Now this is a tool based on their guidelines. Some of the features provided by the tool are already provided by other tool like it dis-assembles .net assemblies.
There are some other features which are really unique and I have not seen been provided by any other tool. Like it helps comparing two assemblies (may be with different versions) and shows the differences in different color. It also allows to document the API is Word. So saves the designer from manually documenting the API once he has really coded it in VS.