There's phpmyfaq which looks more like the specific tihng you have in mind, but I really think a wiki is a better idea. Mediawiki is the best choice if you don

There's phpmyfaq which looks more like the specific tihng you have in mind, but I really think a wiki is a better idea. Mediawiki is the best choice if you don't need to do funny stuff like setting up closed groups. But like jacuesm said, the trac wiki is also nice but only if you intend to use the other trac capabilities IMO.