fp.board is a bulletin board software solution designed to both implement features of proprietary non-free solutions and provide exceptional security. It supports features such as a built in Markdown Engine along with BBCode, an advanced Ban Control Panel (modeled after IRC, with Account Exceptions and CIDR based IP bans), DNS Blacklist checks, checks against the StopForumSpam databases, and more. It has no issues with the Suhosin Patches, and is developed on a server that supports Suhosin.
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.