Microsoft released AJAX 1.0 as an extension of ASP.NET 2.0 called ASP.NET 2.0 AJAX Extensions and now announced they are giving away source code of AJAX 1.0. This is a fantastic news for AJAX and ASP.NET developers. Now developers can not only see how AJAX controls are developed, they will also be able to debug these controls.
There is an AJAX Toolkit called ASP.NET AJAX Control Toolkit available which is a shared-source community project consisting of samples and components. The Toolkit provides both ready-to-run samples and a powerful SDK to simplify creating custom ASP.NET AJAX controls and extenders.
Check out more details here: http://ajax.asp.net/downloads/default.aspx?tabid=47