CuriousWall is a simple PHP forum, with a clean UI.
Import test.sql to MySQL. Then edit connect.php and you are ready to go.
Check https://github.com/cheshirecats/CuriousWall/network before working on it.
Feel free to contribute. Probably this open-source version can be a featherlite forum suitable for individuals and small groups.
FULL VERSION: https://github.com/cheshirecats/ProjectIvory
FULL VERSION DEMO: http://pivory.com