bbPress Thanks is a WordPress plugin that works with bbPress forums and it’s inspired by the popular vBulletin Thank You extension. It places a text area field in the forms used to create a new topic or reply or edit an existing one. Content entered here is hidden from users and only visible when they thank the content, click a Thanks button. The user who authored the content and moderators can always see the content.
Demo: http://codecanyon.net/item/bbpress-thanks-wordpress-plugin/7119265