Vanilla Forums is an open source forum application and discussion framework. Vanilla Forums was initially released in 2006 and today powers discussion on over 500,000 sites.
Vanilla Forums is an open-source, pluggable, themable, multi-lingual community-building solution. It is specially made to help small communities grow larger through SEO mojo, totally customizable social tools, and great user experience. Vanilla is also built with integration at the forefront, so it can seamlessly integrate with your existing website, blog, or custom-built application.
-Vanilla Forums developer