Basic forum is an easy to set up free forums addon that works with concrete5.5 and higher versions. It is very lightweight, and has minimal features. If you need a more comprehensive forum solution, the forum by the core team is excellent.

This forum is intended if you need a simple forum to get your site started. It features a simple form to add a post, and then uses the guestbook block to allow users to reply to a post. A list of posts are provided by a special template for the page list block. 

If you have time to contribute to this addon on github, its very appreciated!

Icon from dryicons.