With the success of maps.google.com intrest has begun to grow in Ajax programming. Functionally AJAX programming JavaScript, DHTML, and XML to “communicate” between a web browser and a server without the need to refresh the screen (actually AJAX is about a great deal more than that, but this is the part that everyone seems interested in.)
To get a good intoduction into AJAX I have found a couple simple tutorials. Using AJAX is a basic tutorial (with examples) for getting web developers familiar with using and developing AJAX. Guide to Using XMLHttpRequest is a baby steps approach to setting up the communications interface between JavaScript and your application server. Finally, be sure to check out Dojo Toolkit, probably the most mature AJAX based API currently available. It dramatically simplifies the process of developing AJAX applications.