Scott Guthrie and the Atlas team released the March CTP of Atlas (the Microsoft implementation of Asynchronous JavaScript and XML, or AJAX) at MIX06.
Atlas has a huge developer productivity story, running inside Visual Studio .NET. It also runs in Visual Web Developer Express Edition (which is free). Atlas provides developers with a way to enhance existing web applications with client-side controls and code execution.
Atlas is not just 'another' script library. Atlas is an development ecosystem, complete with samples, help files and an active developer community.
Get the Atlas March CTP.