Lucid Fried Eggs is a PHP/MySQL application found on http://www.memes.net .
It was developed in response to a CoSource? ( http://www.cosource.com ) request for an open source version of The Brain ( http://www.thebrain.com ).
It's basically a collaborative collection of notes organized in a semi-hierarchical fashion. Each note can connect to any other note as a parent, child or jump link. Any note may have more than one parent, which makes this somewhat different from a traditional hierarchy.
It's a little different from Wiki in that the navigation of notes is very structured. Links and relationships are identified to give the reader a better understanding of the entire structure.