This is an addin for Red Gate Reflector that allows you to compare two assemblies and view the differences between them.
Usage & Licensing
Licensing of Reflector.Diff is free. Unfortunately, since it relies on the Palantir Difference Engine, we cannot release it with source code.
Articles about Reflector DiffWorking with Reflector.Diff
Scott Hanselman - Managing Change with .NET Assembly Diff Tools