The CodePlex description is as follows .....

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.

What's exciting about this to me is how MEF creates a defacto standard for applications that need a "plugable" component architecture.

Think about popular CMS or blogging applications and supporting standard "modules".

For commercial applications, MEF makes designing applications to be sold as a-la-cart easier to do (and you don't have to define a unique plug-in architecture.

Check it out at - http://www.codeplex.com/MEF