The ASP.NET AJAX Control Toolkit is an open-source project built on top of the Microsoft ASP.NET AJAX framework. Its is joint venture of Microsoft and ASP.NET AJAX community to provide powerful platform to build reusable , customizable, and extensible controls. This provides a platform to create a rich media applications.
The AJAX Control Toolkit contains more than 30 controls that enable you to easily create rich, interactive web pages.
ASP.NET AJAX toolkit is available free to download on ASP.NET site.