The ASP.NET 3.5 Extensions Preview provides a new functionality to ASP.NET 3.5. This is the brief introduction to the new features (quoted from www.asp.net).
"The ASP.NET 3.5 Extensions Preview provides a glimpse of new, powerful functionality being added to ASP.NET 3.5 and ADO.NET next year. This release delivers features that enable high-productivity data scenarios and creates the best server for rich clients. The release includes an ASP.NET model-view-controller (MVC) framework, ASP.NET Dynamic Data, Silverlight controls for ASP.NET, ADO.NET Data Services, an Entity Framework runtime, and new features for ASP.NET AJAX."
You can download the enxtensions directly from here.
Microsoft also provided the quickstarts documentation that you can read online from here.