I use "Flash Chat" on my site. I have mine tied to my forum user id's. So you have to be signed up to use it.
It's been live for close to a year now with zero issues. It has many user customizable features. I highly recommend it!
Here is the link to my site, but you have to be logged in to use the chat room as i stated above.
www.sweptline.org
Here is the link to the site I bought it from $5.00! Can't beat that! I did pay the guy to integrate it into my site though, but it was well worth it!
http://www.tufat.com/script2.htm