bbPress

demo: http://bbshowcase.org/forums/topic/new-bbpress-plugin-wiki-post

If you edit wiki-post.php after install, you must reset the plugin via your-forum.com?reset_wiki_post (until there is an admin menu)

There is a separate level control to create the Wiki and then another be able to edit it - so if you want, you can allow only mods+admin to create the Wiki and then any member to edit it.