I've blogged previously about the SharePoint solution that I created that modifies the web.config file of a web application to enable the ASP.NET Ajax toolkit.
While I was at Tech-Ed I saw a presentation where the presenter needed to add this exact functionality to his web.config file, so he preceded to create a blank ASP.NET Ajax web project just so he could copy the relevant lines from the web.config that was created.
So that has prompted me to package up my solution and put it up on codeplex.
You can find the solution and source code at: http://www.codeplex.com/SPAjaxEnabler/
I've put together a screen cast of how to use the tool here as well.