The Simple Guestbook Captcha is probably the best Guestbook script out on the market. Why you ask, well take for starters besides being extremely easy to install onto your Web site--so easy that all you have to do is upload the files and make three chmods. Of course it features are fantastic. Let's start with the fact it has a built in Captcha that prevents bots from spamming your form. Other great features include a full Password protected Administration Control Panel that will let you edit user posts, along with ban ip addresses, messages per page, e-mail notification, time before next post, word ban list, and many other features that will give you and your website a pleasant experience.
Requires PHP on Server