Webfroot Shoutbox allows users to post little blurbs on your website like a small guestbook/chat box/tag board. It is based on what used to be aftertalk.com. Inline or use an IFRAME. Choose either a CSV text file or mySQL database. Simple to set up, and easy to customize colour and layout using CSS, with emoticon parsing option. 11 emoticon themes to choose from, language pack support, admin pages to edit/delete and configure.
A working administratable demo and list of other shoutbox sites available at the website.