I just spent an unhealthy amount of time trying to figure out this error:
Unknown server tag 'asp:ScriptManager'.
I was baffled because I already had a reference to AjaxControlToolkit.dll in my project, and I had what I thought was a web.config ready to handle any AJAX requests. It turned out adding the section below to system.web fixed the problem.
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />