patGuestbook is a PHP-based application described, in the author's words, as "a versatile guestbook with a wealth of configuration options". Developed by Stephan Schmidt, it is freely available for download and is packaged as a single PHP class which can be easily included in your application.
Some of patGuestbook's more interesting features include quick installation and configuration, easy integration into any Web site, multiple guestbooks, easy administration of guestbook entries, automatic email notification of new guestbook entries...the list goes on. If you're the fiddly type who likes to play with advanced settings, the program comes with lots of those, too - ratings, customization of the interface using skins, moderator mode for guestbook entries, and support for so-called patExtras.
Written as a PHP class, patGuestbook can easily be integrated into any PHP-based Web site, and can substantially reduce the amount of time you spend coding guestbooks. You'll find it functional, powerful and (if you're the kind who likes fiddling) easy to customize, and it'll soon be a standard component of every Web site you develop.
Before proceeding further, you should visit the patGuestbook home page at http://www.php-tools.de/ and download a copy of the latest version. The package contains the main class file, documentation outlining how to use it, and some example scripts.