From NETTUTS.com there's a new tutorial they've posted showing you how to create a simple web-based chat application (real-time, not like twitter) with equal parts PHP and jQuery.
In this tutorial we will be creating a simple web-based chat application with PHP and jQuery. This sort of utility would be perfect for a live support system for your website.
Full code is provided, both in the post and available for download, and the end result will look something like this. Messages are just stored in a file on the backend, but it could be easily modified to use something like MySQL to keep track of the messages for that session.