Looking for xhtml guestbook [non wp]

I’ve been using wp for my other site and am loving it. However, I want to have a simple guestbook on my other site.

So I’m asking for help to find a good nice guestbook script to use. I wanting something very light weight, ability to embed it into my site (static xhtml), easy to customise (css and xhtml) and nice clean code.

Thank you in advance for help!