How do I install and use the ASP.NET AJAX Control Toolkit in my .NET 3.5 web applications?

<?xml version="1.0"?>
<configuration>
    ...
    <system.web>
        ...
        <pages>
            ...
            <controls>
                ...
                <add tagPrefix="ajaxtoolkit"
                    namespace="AjaxControlToolkit"
                    assembly="AjaxControlToolKit"/>
            </controls>
        </pages>
        ...
    </system.web>
    ...
</configuration>