First off, examine whether you really need/want a guestbook or not. Almost always, they turn out to be a hassle with spam etc.

The last one I used (2 or 3 years ago) I wrote it myself so it would work and look just like I wanted it too.

All that being said, I found these:
http://www.promosi-web.com/script/guestbook/
http://www.phpjunkyard.com/php-guestbook-script.php

-Chris

I know this is the PHP forum,

I have one script written for ASP.net

http://www.i386.com/cwm/guestbooken.aspx

which has a anti-spam catcha

if you want the script drop us a message or something.

Gavin :-)