We have developed a chat plugin for your facebook applications that will allow your application users to all chat together. It’s a great way to enhance the existing user community surrounding your app.

It is super simple to setup, just paste this code into your app and you are done:
<fb:iframe src="http://www.bestsocialgames.com/widgets/fbchat/" frameborder="0" scrolling="no" width="646" height="600"></fb:iframe>

If you dont have FBML page then you can use this:
<iframe src="http://www.bestsocialgames.com/widgets/fbchat/?api_key=[enter app key]&name=[user name ex:Chat%20User]" frameborder="0" height="600" scrolling="no" width="680"></iframe>

NOTE: for the second sample you have to enter your app key and escaped username.

You can see the app here: http://apps.facebook.com/fv-chat (NOTE: the conversation in this chat room tends to get fairly colorful)

Happy chatting!