Extensibility is now a solved problem. With new Managed Extensibility Framework in .NET Framework 4.0 we can all see the end of building monolithic style apps. Are you tired of hacking your app to bits to meet just one more requirement? Do you want to enable third parties to provide add-on value to your apps? If the answer to any of these is yes, then Managed Extensibility Framework is for you!
For more information, check out this course on Microsoft Virtual Academy: