List of features:
Option to let only registred users shout
Version 1.6 is now compatible with Joomla! 2.5. Note that there are different files for Joomla! 1.5 and 2.5.
There's not really a documentation available, so for some issues you have to crawl the different files within the module folder - that's the reason for "only" 4 stars. But hey, it's free and it works!
I noticed a complaint about getting spam with this shoutbox. I don't get any spam in mine. It's not that hard to configure your settings to prevent this:
You can set the module to be publically viewed, but restrict the shouts to registered users. That's the first step.
Next, even if you don't want to use it, I recommend installing Community Builder along with the CBRecaptcha plugin (set to appear on BOTH the registration page AND the login reminder page.
I used to have spam problems. Now that I'm using CB and Recaptcha, even with the shoutbox set to be viewed by guests, I don't get ANY spam anymore.
Use Recaptcha and restrict any posting areas of your site to registered users, and the spam will virtually disappear.
Even in the shoutbox!