When I was coming back from Mix 2007, I was bored on the plane and so started to write a dev tool. What a geeky thing to do on a plane. :-)
The tool allows comparing two versions of an assembly to identify API differences: API additions and removals. Comparing versions of APIs comes very handy during API design process. Often you want to ensure that things did not get removed accidentally (which can cause incompatibilities), and as APIs grow, you want to review the addition without having to re-review APIs that were already reviewed. The tool, called Framework Design Studio (FDS) supports these scenarios.
Later on, I got lots of help from Hongping Lim (a developer on our team), and David Fowler (our 2007 summer intern). David ported the application to WPF, and Hongping basically took it from an early prototype stage to what it is today and made it possible to ship it externally.
Anyway, you can get the tool at the MSDN Code Gallery, the user guide is attached to this post, and lastly, here is the API diff output that the tool generates. Hope you find it useful.