Please take a look at the following articles related to the v15.1 release:
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Ajax Control Toolkit Documentation
The Ajax Control Toolkit contains a rich set of controls that you can use to build highly responsive and interactive Ajax-enabled ASP.NET Web Forms applications.
Note: This content was moved from
http://www.asp.net/ajaxlibrary/act.ashx to this Documentation wiki. This is now the authoritative location of the Ajax Control Toolkit documentation.