The Managed Extensibility Framework (MEF) is a new feature of .NET 4 (and will work on 3.5 as well) that makes it significantly easier to extend applications and generally support application composition patterns.
What does that really mean? Well, take 6 mins and 5 seconds to watch this excellent demo Scott Guthrie did at his PDC2008 keynote.
Download video directly
Other MEF resources: