In .NET 3.5 Microsoft created a set of guidelines and libraries specifically for creating extensible applications. Often referred to by its namespace identifier, System.AddIn gives developers the tools they need to support third-party extensions without jeopardizing the security and robustness of their application as a whole.
Much of this is accomplished through the use of AppDomains and rigid communication pipelines. However, building all of the boilerplate needed to make this work is rather daunting. Hence the release of the Pipeline Builder. But while it does give developers a leg up, it has no way to warn them when they are doing something wrong.
This is where a new set of FxCop rules for AddIns come into play. While they certainly won't catch everything, they should point developers in the right direction. These rules are only available in source code format and fall under the "Microsoft Limited Public License".