The Anthem project adds AJAX-like features to ASP.NET.
What sets Anthem apart from other frameworks is that it integrates itself into the server-side control model made popular by ASP.NET. This includes full support for view state, server-side events, and everything else the typical ASP.NET developer has grown accustomed to.
To get the latest release, download it from the list of releases on SourceForge.
Please note that you need to be a fairly experienced ASP.NET developer in order to use Anthem to its fullest. There are, though, dozens of examples included in the release to help get you started.