Chances are that if someone only runs one CGI script, it'll be a guestbook. They're one of the easiest scripts to code, and one of the best scripts to use since it provides a type of registry of viewers to a given page.

Book Of Guests - By Dream Catchers
Your basic guestbook. Admin can setup the script to his or her own likings with many different options. Script can email the admin with new entries as well as email a thank you to each signer. No demo available.

Comments Script - By Matt Kruse
A nice looking guest book that has lots of extra options. Great admin feature lets the admin edit the guest book to delete older or bad entries. Search feature also adds a nice touch to it. Demo on main page.

Graffiti Wall - By Virtual Marketing
Nothing really more than the others, just includes some graphics to make it look more like an actual "wall" that people are putting "graffiti" on. People can only leave their name, email address, and comments. Click for demo.

Guestbook - By Matt Wright

Nice guestbook that has many features and options. Allows people to leave their name, email address, web page, comments, city, state, and country. Click for demo.

Matt's Source Code - Guestbook - By Matt Leonard
Another basic guestbook. Allows for entry of all the basic information... name email address, etc. Demo on main page.

Registry - By Dave Palmer
Has a nice looking output which takes advantage of different fonts. People that sign the guestbook have the option of viewing their post before actually posting, or if they choose they can just have it automatically added. Click for demo.

Selena's Guestbook Script - By Selena Sol
Modified from Matt Wright's guestbook, offers a few more options, such as giving the admin the option to allow or disallow HTML tags. Also has the ability to censor out words. Looks nice, with comments indented. Click for demo.