What is Ajax ?
What is Ajax ToolKit?
The Ajax Control Toolkit contains a rich set of controls that you can use to build highly responsive and interactive Ajax-enabled Web applications. The Ajax Control Toolkit contains more than 40 controls, including the AutoComplete, CollapsiblePanel, ColorPicker, MaskedEdit, Calendar, Accordion, and Watermark controls. Using the Ajax Control Toolkit, you can build Ajax-enabled ASP.NET Web Forms applications by dragging-and-dropping Toolkit controls from the Visual Studio Toolbox onto a Web Forms page.
Download Link :-
You can download the latest version of the Ajax Control Toolkit from Asp.Net.
If you are using Visual Studio 2008 then you should pick the version of the Ajax Control Toolkit for .NET 3.5. If you are using Visual Studio 2010 then you can use either the .NET 4 or .NET 3.5 versions of the Ajax Control Toolkit.