Anthem.NET is a free, cross-browser AJAX toolkit for the ASP.NET development environment that works with both ASP.NET 1.1 and 2.0.

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.

Introduction to Anthem.NET - The Code Project - AJAX / Atlas