Planet Minibox gives you an embeddable shoutbox you can put anywhere on your blog or Web site. Visitors can leave a message or strike up a conversation with others in real-time. Think of it as a mash-up of comments, instant messages, and a full-fledged forum. What separates a Planet Minibox shoutbox from others is the tabbed interface that lets you strike up private conversations with another user simply by clicking their name.
Despite its fairly simple user interface, the administrative back-end of Planet Minibox is quite advanced. If you're comfortable with managing user accounts, your Planet Minibox doubles as a bona fide forum. Users can register, be appointed as moderators, get their own avatars, and keep track of message history. Or you can keep it simple and just give people a place to chat without having to register.
You can easily customize a Minibox by picking from a dozen or so colored templates. Alternately, you can go through each element of your Minibox and select the font and color. This is handy if you want to design a Minibox that matches your site's color scheme, but I wouldn't recommend it for the casual user.
Shoutboxes like Planet Minibox's make a neat alternative to using forums or an IRC client to allow your users to talk to each other. In some cases, it's just a pain to integrate either of those two options into your site or blog. Where comments can provide basic feedback and discussion, instant messages make for a more dynamic conversation experience. See also YellBox, ShoutMix, or Twitter, the last of which takes the concept to mobile phones.
[Found on Museum of Modern Betas]