Krzysztof Cwalina announced that Microsoft has released a CTP of its Managed Extensibility Framework (MEF).
MEF is basically a dependency injection framework that allows to provide extensibility for your applications (add-in/plug-in framework). Also, it's a very good way to componentize your application's architecture and make components/layers pluggable and replacible. Here's a very good overview of what MEF actually is with nice examples and excellent comments from readers: http://blogs.msdn.com/kcwalina/archive/2008/04/25/MEF.aspx
Go grab it here: http://code.msdn.microsoft.com/mef. The download contains MEF itself (ComponentModel.dll) and several interesting samples.