Download Prism 4.1 at Microsoft Download Center
Prism provides guidance designed to help you more easily design and build rich, flexible, and easy to maintain Windows Presentation Foundation (WPF) desktop applications and Silverlight Rich Internet Applications (RIAs) and Windows Phone 7.1 ("Mango") applications. Using design patterns that embody important architectural design principles, such as separation of concerns and loose coupling, Prism helps you to design and build applications using loosely coupled components that can evolve independently but which can be easily and seamlessly integrated into the overall application. Such applications are known as often referred to as composite applications.
NOTE: If you are interested in Prism, you may also take a look at this ongoing SilverlightShow article series (4 parts available as of 2/20/2012) by Brian Noyes: "Working with Prism 4".