On freewebs you can use the same code for your chatbox as you use on your forum. That way you can be on one site or the other and still use the same chatbox. If you use "CBox", there is also a direct link that people can use so the chatbox opens in a seperate window allowing them to chat in that same chatbox and not be on either site. That link is on the main Admin page on the cbox website.

You can use IRC to make a chatroom.
Windfyre.net offers a free Java applet.

http://www.windfyre.net/javachat/source.php
Copy the "Quick Windfyre Site Link"

You might want to get familiar with IRC though before you try to use it.

Is there a link to the chat? You chould just have a clickable link that opens in a new window.