The AJAX Controls Toolkit is an open source toolkit developed by Microsoft and the ASP.NET community. The latest version is for .NET 3.5 and it contains 38 client side components used to create Rich Internet Applications.
The AJAX Controls Toolkit can be downloaded from http://www.codeplex.com/AjaxControlToolkit. There are four downloadable zip files, download AjaxControlToolkit-Framework3.5SP1-NoSource.zip if you are not going to modify the source code. Extract the downloaded zip file to a folder. Start Visual Studio and create a new ASP.NET Web Application or open an existing one. Right-click on the Toolbox and select Add Tab from the context menu to create a new tab. Give the tab a descriptive name and right-click on it and select Choose Items... from the context menu.
The Choose Toolbox Items dialog box will appear. Select Browse, go to Bin in the folder you just extracted the toolkit to, and select the file AjaxControlToolkit.dll and click Open.
Now the DLL file is registered and the components should appear in the Toolbox.