Recs.
Updated
Discourse is a Civilized Discourse Construction Kit, a free, flexible, fundamental building block for online discussion communities of any size, tiny to huge.
SpecsUpdate
Pros
Pro Has a built-in immune system from trolls, spammers and bad actors
Discourse has a simple user trust system that makes moderating the forum a lot easier. Users gain more permissions as they gain more trust, which limits the amount of damage spammers and trolls can do. Discourse co-founder Jeff Atwood also founded StackExchange, which is the gold standard for gamification/moderation systems so you can also expect Discourse to get better and better at moderation.
Pro Modern & polished UI/UX with lots of great smaller features
The best feature is its design: discourse is designed to aid in the creation of high quality online conversations. Flat discussions with well implemented quoting systems, expandable/collapsible replies, infinite scroll, clean UI and many more features come together to form a highly polished forum experience that makes legacy forums show their age.
Pro Helpful community
Discourse has a comprehensive forum culture support hub at meta.discourse.org.
Pro Hosted and self-hosted solutions
You can host it yourself in a $5 cloud server or on any Linux server you already have. And if you don't want to deal with this, you can use the hosting platform from the development team.
Cons
Con Complicated setup
Compared to standard PHP+MySQL apps, Discourse is more complicated - the app lives in a Docker container. However, a Docker image is available, which reduces the setup time to about 30 minutes.
Con Extremely expensive hosting
Official discourse.org hosting starts at $100/mo and supports the team creating the software. It can also be hosted on a cloud service like Digital Ocean for $5/mo.
Recommendations
Comments
Flagged Pros + Cons
Con Un-professional Co-founder
One of the Co-founders, Jeff Atwood, can be very unprofessional and even violates the terms of service on Discourse's own support site. Any time you try to call him on it, the evidence gets deleted and you get suspended from the forum.
Con Only free if you host it on your own
If you don't want to manage your own web server, you get a demo version to try out for 14 days and after that, you have to buy any of their three versions.
Out of Date Pros + Cons
Con Battery drain on mobile is extreme
Not only can Discourse drain a mobile phone battery in under an hour, it pushes the hardware so hard the phone heats up to the point where it's uncomfortable to hold.
Con No themes beyond changing colors
Discourse doesn't have a themes ecosystem. All you can do is change the colors.