I get a lot of hits on this article I posted last year, showing how you can use ASP.NET Control Adapters to automatically attach Control Extenders to a specific kind of ASP.NET control throughout an existing web site, without modifying any of the existing pages.
It uses a Control Adapter to create and attach extenders to the target controls, and also an HttpModule to ensure that all pages have a ScriptManager or ScriptManagerProxy.
Anyway, I finally got around to uploading a demo project showing this. It is slightly different to the article, since it shows how to attach a DropShadow extender to all UpdatePanels on your site. Not something you will necessarily want to do, but you get the point.
The demo project is here. It consists of a class project containing the Control Adapter and Http Module, and then a sample web site with a .browsers file to cause the adapter to be used, and a modified web.config to let the Http Module do its stuff.