A free guestbook service, with a number of customizable features. For instance, you can add a graphic, or change the font face and size. For a one-off payment of $10, you can upgrade to "VIP" level service (no banner ads, plus more storage space for messages)
A very fast guestbook service, and easy to set up. However, their advanced mode basically "pops the hood" since you can edit most of the HTML on the guestbook page directly so that it will be 100% customized for your site. A banner ad is shown on each page.
This service provides elegant, fast loading guestbooks that can be customized in many ways. Features include profanity filtering and IP and domain filters to limit access to your guestbook. There is a limit of 1,000 messages in the free version of the service.