Vanilla is an open-source, standards-compliant, multi-lingual, theme-able, pluggable discussion forum for the web. With over 450 plugins so far, Vanilla is ideal for custom community solutions. Anyone can download and use Vanilla for free. If you do not have a web space, you might be interested in the hosted version of Vanilla as well.

Vanilla lets you have total control over the appearance of your forum so it can completely mesh with the look & feel of your site. Vanilla is fast. Setting up a forum takes moments, using Vanilla is a breeze. With Vanilla’s Pages feature, you can add custom pages to your forum and turn it into a complete website. Add a bunch of plugins and you’ve got a totally interactive website to go along with your forum.

Requirements: -
Demo: http://vanillaforums.org/
License: GPL License