GoatBB
Goatbb is an easy to use open source forum, this means you can download and use it for free.
GoatBB uses Bootstrap as layout framework so that you can easily change the layout to what you want without having to worry about making the layout responsive. For the BBCode editor GoatBB uses Summernote which is a bootstrap based WYSIWYG editor so that makes even the editor responsive and functional on phones and tablets.
Website:
https://github.com/MrGoatsy/GoatBB