Michael Schwarz has released the source code for his Ajax.NET library. Read more about downloading Ajax.NET here.
He also has a PowerPoint Presentation that provides a quick overview of Ajax.NET and how to start using it. This is a great presentation for a .NET developer group. Download the presentation here.
Note that although AJAX is cool, it really sucks with search engines. If search engine optimization and ranking is critical to the success of your website, you may want to avoid AJAX altogether.