AjaxCoded

Here is a nice little tutorial on using Ajax to create drop down menu boxes. Adam does a good job of walking through the easy development of this and supplies us with samples and source code.
In our example, we’ll use a pair of dropdown menu boxes (SELECT tags in HTML). A selection in the first box affects our list of choices in the second box. It’s not exactly cutting edge (Thau did something similar using JavaScript), but it’s a proof of concept.
Link : Nitty Gritty Ajax: by Adam DuVander 12 Apr 2006

Although there are a lot of tutorials that have the same basic syntax and characteristics it is educational to look through as much as you can.
Every programmer has their own unique way of writing and documenting (or not) code and sometimes one particular style or structure may strike you differently and open up something that was not apparent before.
With that said, here are a few more tutorials to check out.
Link : DP- AJAX Tutorials