PunBB - PunBB is a fast and lightweight PHP-powered discussion board.
Forumbee - Forumbee offers community forums for discussions, feedback and Q&A.
Vanilla Forums - Build an engaging community forum using Vanilla's modern cloud forum software.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
FUDforum - FUDforum is a free and open source Internet forum software
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.