Vanilla is a light weight Internet forum package written in the PHP scripting language. It is released under the GNU GPL.[1] Vanilla Forums are open-source, standards-compliant, customizable discussion forums.
Vanilla is a free, open-source, pluggable, themable, multi-lingual community-building product to host a forum on your own server infrastructure.
Some live Vanilla forum example which looks "wow" : http://css-tricks.com/forums/
For more detail visit http://vanillaforums.org/