I wrote this specifically for this website; originally written in Perl, eventually converted to php.
The guestbook is stored as a flat file on the server. As well as the users' comments, their ip address and the time of the entry is logged. When a user adds a comment it is immediately displayed (unless moderation is enabled), and an email is sent to a predefined address (if so configured).
There are also a few (optional) techniques employed that attempt to limit spam.