Krzysztof Cwalina: Framework Design Studio
If you are building and evolving APIs, Framework Design Studio is a tool you should check out. Krzysztof announces it here.
(the app is built in WPF...)
Another post that may be of interest to you
- FxCop Rule for Multi-Targeting - build an app for .Net 3.0 using VS2008 and you may accidentally call some new APIs in WPF that didn't exist in 3.0 (and will only fail at runtime.) Use an FxCop rule to detect this!