free php live chat with visitor/customer script

I was recently visiting a commercial site and suddunly a window chat popped up and some assistant asked me if I needed some help. I thought this was pretty cool and I was wondering if such a thing existed in php, i know there are lots of php live chat script out there but I'm not sure if they allow to monitor visitors on your webpage and launch a window chat to the visitor of your choice. does anyone know of such a thing in PHP? I think the application on that site I visited was a java applet. thanx in advance Pat