The ASP.NET 3.5 Extensions Preview is a new release that provides new functionality being added to ASP.NET 3.5 and ADO.NET in 2008. This release delivers a set of new features that target:

1. Enabling High Productivity Data Scenarios - including MVC, Entity Framework, ADO.NET Data Services and dynamic data
2. Supporting Test Driven Development - including a powerful and extensible MVC framework
3. Creating the best server for Rich Clients - including Ajax history support and Silverlight controls for ASP.NET readmore