WSN Guest is a flexible PHP/MySQL guestbook script distributed under the free and permissive MIT license. Most of the development took place in 2003, but it continues to function well today. Take it and make commerical derivative works if you like. Make a post on the forum to share your improvements or sell your derivative product.
Requirements: PHP 4.1.0+ (or PHP 5), MySQL 3+. A knowledge of HTML and CSS is very useful for taking full advantage of the template system.
Last version: 1.25, released May 2014
Documentation: See readme.txt.
Demo: Launch WSN Guest demo