As reported over on AjaxDeveloper.org:

Rasmus (of PHP fame) has shared a simple "30 second Ajax tutorial" as reported here.

I was reading through the mails in the php-general mailing list and came across this mail by Rasmus about AJAX:

"I find a lot of this AJAX stuff a bit of a hype. Lots of people have been using similar things long before it became 'AJAX'. And it really isn't as complicated as a lot of people make it out to be. Here is a simple example from one of my apps."

He then steps through a good little (and yes, very simple) example of how to make a successful Ajax connection - in this case, to update the contents of a DIV tag.

It's light. It's clean. It's simple. Doesn't get much better than this...