Crafty Syntax Live Help is an open source chat-based help system.
From the Author
Live Help is a multi-user, multi-operator, multi-department live Help support chat system that allows the operators of the websites to monitor their visitors as they are browsing the site and proactively open a chat session with the visitor.
Features include auto-invite, referer tracking, page tracking, chat notification, user is typing message, multiple chat sessions, sound alert, leave a message if offline, push urls, quick responses, and multiple operators.
Author website: http://www.craftysyntax.com