Microsoft .NET Framework version 3.5 is now available for Windows Vista, Windows XP and Windows Server 2003. Though this release is a Community Technology Preview (CTP) release yet it gains a lot of significance since the Microsoft .NET Framework 3.5 contains many new features that are compatible with both .NET Framework 2.0 and .NET Framework 3.0. .NET Framework 3.5 also includes servicing for .NET Framework 2.0 and .NET Framework 3.0. .NET Framework 3.5 has been build incrementally on the new features added in .NET Framework 3.0, for example, Windows Workflow Foundation (WF), Windows Communication Foundation (WCF), Windows Presentation Foundation (WPF) and Windows CardSpace. In addition, .NET Framework 3.5 contains a number of new features in several technology areas. These new features have been added as new assemblies to avoid breaking changes which I guess is totally cool.
The 3.5 version also introduces a new Web protocol support for building WCF services including AJAX, JSON, REST, POX, RSS, ATOM, and several new WS- standards. And full tooling support for WF, WCF, and WPF, including the new workflow-enabled services technology and new classes in .NET Framework 3.5 base class library (BCL) which I guess is gonna address the most common developers requests.
You can download .NET Framework 3.5 Beta from here.