FluxBB is a fast, light, user-friendly forum application for Web sites. It was originally based on PunBB and supports UTF-8, XHTML, and internationalization, and leaves out some of the less than essential features of other forums without sacrificing essential functionality or usability.
TinyIB is a lightweight PHP image board which emulates the functionality of 4chan. If you use MySQL or SQLite, you can use it to create an efficient setup able to handle large amounts of traffic. If you don't use a database, it can store posts as text files for a portable setup capable of running on virtually any PHP host.