Krzysztof has blogged about the Managed Extensibility Framework (MEF) a while ago. Today, the Application Framework Core team has published the first Community Technical Preview (CTP) of MEF!
For those that are not familiar with the topic, MEF "provides
developers with a tool to easily add extensibility to their
applications and with minimal impact on existing code. The application
developer can define extension points according to the functionality
required of an extension, while the extension developer uses those
points to interact with the application.
MEF enables this
extensibility to take place without creating a hard dependency in
either direction. Applications can be extended at run time without
recompilation, and extensions can be used by multiple applications
sharing the same extension requirements. MEF also allows an application
to delay the loading of an extension while still examining its
metadata, enabling efficient traversal of large catalogs of extensions."