ZK is an open-source Ajax Web framework that enables rich user interface for Web applications with no JavaScript and little programming.

Sajax is an open source tool to make programming websites using the Ajax framework — also known as XMLHTTPRequest or remote scripting — as easy as possible. Sajax makes it easy to call PHP, Perl or Python functions from your webpages via JavaScript without performing a browser refresh. The toolkit does 99% of the work for you.

ATLAS is  free framework for quickly creating a new generation of more efficient and interactive Web experiences that work across all the most popular browsers. With ASP.NET AJAX you can create next-generation interfaces with reusable AJAX components, enhance existing pages using powerful AJAX controls with support for all modern browsers. Continue using Visual Studio 2005 to take your ASP.NET 2.0 sites to the next level. Access remote services and data from the browser without tons of complicated script.

Ajax.NET Professional is one of the first AJAX frameworks available for Microsoft ASP.NET and is working with .NET 1.1 and 2.0. You can find here a quick guide how to get the first Ajax.NET method running.