Orphereus is a modular imageboard engine with a broad set of features: a flexible system of tags and filters, a pluggable architecture, memcached support for better performance, full-text search powered by the Sphinx engine, a private mode where invitation is required to join, AJAX, extended security, highly customizable appearance, an improved analogue of tripcodes, news and statistics modules, advanced administrative, a maintenance subsystem, internationalization, and much more.
Sukiyaki was born out of what the author sees as a much needed improvement to current imageboard/BBS software. They're flaky, often have security flaws that users take advantage of, and worst of all, they have the same UI from the mid-nineties. It's time for an update! Sukiyaki visually organizes replies in threads so that it is clear who the reply is sent to. It is written using Django with its wonderful ORM, which helps reduce security vulnerabilities. It has been tested in production and is stable. It also includes a unique system for user-ranking of posts. Planned features include RSS updates for individual boards and threads, and there is a faster backend (Django-Treebeard).
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.