We use shoutbox, which is ok but does weirdly sometimes censor some words!
Its a bit basic, but free. Actually has been ok, been using for 2 years.
You need to delete the youtube links from the source code as the chatbox links to a youtube channel for the widget builder making origami cranes (?!)
Heres the code page:
https://shoutbox.widget.me/
our page here:
http://www.bidstonobservatory.org/radio
