This is interesting as I do now use some Ajax code on ParadoxalPress.com. Currently I only use the MagicAjax library but plan on switching to Atlas once it gets closer to be released...
The "Atlas" Control Toolkit is a set of nine great controls and extenders that use "Atlas" technologies and allow developers to easily improve the client experience on their websites. All of the controls come with full source, and the toolkit also includes Visual Studio 2005 templates to get you started writing your own controls.
The toolkit includes an easy to use SDK that simplifies the process of writing "Atlas" components. It also has a rich set of sample controls and extenders that make spicing up your Website with rich functionality a snap. Everything you need to get started is inside including full source code, documentation.
For details check out the site at:
Landing Page: http://atlas.asp.net/default.aspx?tabid=47&subtabid=477
Toolkit samples live: http://atlas.asp.net/atlastoolkit/