Here are other free forum software (excluding hosting):
- Simple Machines Forum: http://www.simplemachines.org
- phpBB: https://www.phpbb.com
- MyBB: https://mybb.com
- Vanilla Forums: https://vanillaforums.com
Also, usually any web hosting with cPanel will come with one-click forum installation scripts, which you can install and try out. I personally would still go for the $20 discourse hosting above though.