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Set which fields are required, Select how many messages to show per page, Edit the How you found us list, Spam protection (1 message per IP every 24 hours).
Price: Freeware

Guestbook is a classical script who visitor can write some word and leave comments on your site.
Price: Freeware

Your visitors can leave messages for you, for each other, or even use your guestbook as a basic message board. Installation of YouGuestIt is a breeze, just upload it and go.
Price: Membership starts at $29.95

this guestbook was designed to be implemented into your current site design, which should be very easy.features:no mysql,fantastic support for large files,ip logging,max/min post length,max name length,admin area to edit/delete posts,posts are 'archived' to other pages,only ever uses 2 files, the guestbook.php and the file which stores the posts,100% secure,post time limit.
Price: GPL

12react can be used to offer visitors of your site the ability to react to certain things on your site. These things can be news-items, pictures, you, etc. The script can handle multiple reaction pages but as reaction page to your entire site it could also function as a guestbook.The script comes with an extra script that easily installs the MySQL database for you. Apart from the database login parameters, you don’t need to do anything else. The script uses a text variable and can be easily changed into another language. Default language is English.
Price: Free

A PHP guestbook script featuring: email notification to the admin for each new message, extensive functionalities for moderating records, ability to reply to any entries, flexible adjustment through web-interface, filter for bad words and tags, uses QIM (Quick ICQ Message). No SQL database required.
Price: Freeware

4CM_GuestbookPro is a powerful PHP and MySQL driven guestbook. It will allow your visitor to list their Name, Email, URL, AIM, ICQ, YIM, Location, a national flag and any comments they may have. Additional features include: Private Messages, Extensive field verification, min/max character limits, bad word censoring, invalid name censoring, IP banning, Search Engine, Admin Email Notification, Administration Replies, Page Spanning for large guestbooks, Language Translation System, Anti-Double-Post Via IP Address, long-line (sticky line) word-wrap feature, "time-zone" feature, Alternating Comment Background Color, Email Anti-Leach Feature, Full Control of the Appearance of the pages, the dreaded Smilie Feature, and much more. Included is an Administration script to fully administrate the entries in your guestbook as well as an installation script to automate the installation process! The latest update includes: A search engine, national flags and compressed code.
Price: $15

This is a PHP guestbook script that utilizes a new technology called EBS (developed by @lexPHPteam) which offers the comparable advantages as using such database as MySQL. Features: automatic installation, 2 languages (French/English), password-protected administration, 100 % configurable and adaptable to your site via forms, and entry management.
Price: Free (GPL)

Many functionalities: 5 databases accepted + ODBC connection (MySQL, MsAccess, MsSQL, Oracle, PostgreSql 7), automatic installation, smilies, management of the censurable words, IP addresses banning, search engine, etc.
Price: Free (GPL)

A PHP guestbook script that uses MySQL on backend. Features: automatic installation, 2 languages (French/English), password-protected admin, 100 % configurable and adaptable to your site via forms, and entry management of messages.
Price: Free (GPL)