I've gone ahead and added another category to the technologies of interest to WCF developers. This entry covers the Managed Extensibility Framework.
The Managed Extensibility Framework makes it easier to create and reuse extensible applications by providing a model for discovering and composing application plugins. MEF provides a standard way for an application to advertise its extensibility points and consume extensions. MEF also supports tagging extensions with metadata for querying and filtering. This allows you to get an off the shelf component model without having to build the infrastructure yourself as part of your application.