Wikipedia Browser
I ran across this, and it'spretty cool.   This little AJAX application is released under the GPL and allows you to browse the Wikipedia in a friendly little browser style interface.  I'm looking forward to digging through the code to see how it's all setup.  This is definitly a great example of the capabilites of AJAX technologies.

By the way, I have finished Part One of my AJAX Chat Tutorial.  It's pretty basic, and lacking much desired functionality, but if you need a quick start to creating AJAX style websites, this is a good place to start.  All the source code to the application is avaliable, and the tutorial explains the application line by line.  Try it out and let me know what you think.  I will be working on part two later this week.