I didn't want anything fancy, so I made one that's really simple.
Users can add their name, email address, and a short comment.
Then, when the guest book is displayed, all entries are shown in reverse chronological order, meaning the most recent entry is displayed first. All of this stuff would be easy to add. But as always, everything is located in one file. And you don't need any fancy overpowerful libraries to make it work. It's all self-contained.