A shoutbox, saybox, tagboard, chatterbox or chat box is a chat-like feature of some websites that allows people to quickly leave messages on the website, generally without any form of user registration.
In their simplest form, shoutboxes are simply lists of short messages, possibly with information about their authors.

Many Internet forum and weblog software packages can be modified to add shoutboxes in sidebars on site pages. Shoutboxes are usually maintained in a manner similar to that of more complex boards, with moderators that can delete posts and ban usernames or IP addresses. Occasionally, features such as impostor identification, flood control, and profanity filters may be included.

Yes, shoutbox.com is totally free to be used.
Integration cannot be simpler: you just have to paste 1 single line of javascript.