PHPGuest is a very small but very functional guestbook program. It was written so that any webmaster can insert the code into the website with no problems at all, it is 100% xhtml compliant and written in PHP.
Evolution Guestbook is a free guestbook which features private messages, email sending, icq messaging and an adminstrative panel to help you control the guestbook effectively. Also supports 7 diffrenet databases.
Advanced Guestbook is a PHP-based guestbook script. It includes many useful features such as preview, templates, e-mail notification, page spanning , picture upload, templates, html tags handling, smilies, advanced guestbook codes and language support. The admin script lets you modify, view, and delete messages. Requires PHP4 and MySQL.
hwdeGUEST is a Guestbook written in PHP that uses flatfiles (simple text files). It includes an editor utility where you can delete, change and answer to entries. The software is completely HTML template based, making it very easy to include hwdeGUEST in almost any design. Other features: Great email notification system (multiple possibilities), ban HTML Tags from your entries, multilingual due to an external languagefile, and much more.
Harry's PHP Guestbook is a simple guestbook script that uses a flat database. Users can leave a message, their name, homepage and email address. It is template driven, so it can be easily customized for every website.