Website Chat?

I have seen it on other pages, a little chat window visitors to a web site can talk to each other. I do not know if connected by AIM or something so they can use their own names. Does anyone know what this is called? I tried searching the web but using words all too common (live, chat, website, etc) are no help.
Anyone know what they are called?
And if so, where I can download a FREE one? I do not need super duper options and such. Something simple like AIM chatting is. No graphics and such. Maybe those circle smily thingys.
Thanks.