Cornucopt is a PHP/MySQL-based Wiki engine with Everything2-like UI customs. It also features "hard" logins/permissions and an extensible framework that makes it easy to create specialized (e.g. weblog- or forum-like) behavior for a given page.
We are waiting on a couple of key features to be written before we put up a demo Cornucopt site at this URL. In the meantime, enjoy our development weblog, which is comedic in at least a couple of senses.