Here are some written with PHP and JavaScript (not necessarily jQuery):
- Micro Chat
- BlaB! Lite
- PHP Chat
- Free PHP Ajax Chat Super
Specifically jQuery:
- Gmail/Facebook Style jQuery Chat
- Building a jQuery/PHP Powered Chat
- Free PHP Chat Script + Tutorial
There's also a list of 279 PHP chat scripts here, a lot of which seem commercial but some are free.
If you browse enough, I'm sure you can find one to suit your needs.
