Using the Ajax Control Toolkit in ASP.NET MVC
Stephen Walther has a pretty cool post on using the new file-only version of the Ajax Control Toolkit from an MVC application:
http://weblogs.asp.net/stephenwalther/archive/2008/08/22/asp-net-mvc-tip-36-create-a-popup-calendar-helper.aspx
UPDATE: the problem with Stephen is that he writes quite fast. So pretty much in the time it took me to link to his first post, he wrote a second one, this time on using the AutoComplete behavior:
http://weblogs.asp.net/stephenwalther/archive/2008/08/23/asp-net-mvc-tip-37-create-an-auto-complete-text-field.aspx