Vanilla is an open-source, standards-compliant, multi-lingual, fully extensible discussion forum for the web. Anyone who has web-space that meets the requirements can download and use Vanilla for free!
V. Gómez, A. Kaltenbrunner, and V. López. WWW '08: Proceeding of the 17th international conference on World Wide Web, page 645--654. New York, NY, USA, ACM, (2008)