JibberBook is an easy-to-use, open-source guestbook built with PHP5 and JavaScript. It aims to create a pleasant user experience while giving you much flexibility. JibberBook is open source under an MIT license so you’re free to use it where ever you want. It has the following major features.

Spam Protection
Stop spammers in their tracks with extra protection from Akismet. Spend more time doing what you want and less time managing your guestbook without the use of an ugly Captcha box.

Storage Options
If you don’t like the quick and easy default XML storage, you can switch to MySQL. Simply replace a file and fill out your database information.

Usable Interface
Users with JavaScript will experience a user-friendly interface enhanced by the light-weight library, Mootools. The interface gracefully degrades, and functionality will remain the same for users without JavaScript so you don’t miss a comment.

Requirements: Mootools Framework
Demo: http://jibberbook.com/demo/
License: MIT License