go and Google this (make sure to use the " marks too
"Script""guestbook"
That should at least get you started on some. I never had a site where I had a guestbook, only a message/chat board. I always preferred that to a guestbook, but that's just me.
Usually Googling "script" "whatever I was looking for" would turn up at LEAST 2-3 pages of useful links.
Bravenet also had a lot of scripts to use, but I can't remember if they were specific to their site.