Simple Machines Forum, or SMF for short is a free to use forum software that was a branch of another software called Yet another Bulletin Board. The first version of the software was released in 2003, and it's still being developed to this day. It's my favorite free forum software, it's the easiest to set up and install themes as well as plugins in my opinion. I love the way the admin control panel is designed, and the default forum theme looks sleek too. Do you like this software? Do you currently use this software on your community?
I was unsure if SMF was ever still around or being used. I used to be a member on a lot of SMF forums back when I was frequently using forums and I used to enjoy using them as a member but I never looked into them as an admin.
SMF's default theme is mobile friendly. In that case, like phpBB, I assume all the other ones are. Anyway, that's a big plus. Another plus to the software is that newbies can simply upload themes and extensions like on WordPress.