These are the parts of an amazing series by Brett McLaughlin at IBM. This is not a easy/fast read so suggested putting away some time(hour or two) before getting your hands into it.

Gain a complete understanding of HTTP status codes, ready states, and the XMLHttpRequest object
For many Web developers, making simple requests and receiving simple responses is all they’ll [...]

Sematopia.com has post a quick and easy tutorial on learning Ajax in 20 minutes. They have stated that this tutorial only works in Firefox so if you are an IE user you need to add some Javascript to detect which XMLHTTP object to select.

Here is a quick overview and example of AJAX in action. Before [...]

Nuvvo is an ajax/web 2.0 powered web service application enabling creating your own e-learning systems online.. What they said…

Nuvvo is your way to teach on the web. Everyone knows a little bit about something, and this free, AJAX-enhanced eLearning web service is designed to bring out the teacher in all of us. Sign up and [...]