Ajax is not a single technology, but a group of technologies. HTML and CSS can be used in combination to mark up and style information. The DOM is accessed with JavaScript to dynamically display, and to allow the user to interact with the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads. In this post you will find the best websites that can make you an Ajax developer.
Also See
Mozilla Developer Network
ajax.solutoire.com
IBM Ajax Resource Centre
Encosia
Site Point
Ajaxonomy
Ajaxian
Ajax Magazine
Ajax Lines
Ajax Impact
David Walsh Blog