Hmm… One more acronym to remember :)..
“The Managed Extensibility Framework (MEF) is a new library in .NET that enables greater reuse of applications and components. Using MEF, .NET applications can make the shift from being statically compiled to dynamically composed. If you are building extensible applications, extensible frameworks and application extensions, then MEF is for you.” – MEF Site
MEF is a relatively new project which is still in development (alpha release) and looks very promising