Traditional: Discourse, phpBB, Vanilla, Xenforo, IPB, Flarum (lot modern than the rest)
Modern: Circle, Tribe, MN, Playgroup, Forem, Habitate
Not-dedicated: Slack, Facebook groups, Telegram, Discord (built around gaming)
There are a lot more platforms that are not mentioned here, each platform has its own pros and cons. Proper time and effort should be spent on the subject, something I did while evaluating join.coulf.com - the private community I have built for indie product makers and founders.
Slack or a Fb group may sound like the cheapest way to launch your community. It actually is but with * attached.
Slack is a focused work environment mostly preferred by ages 25 and over. It is preferred by community builders because the audience already exists here, one less barrier to launch. Slack may have every feature to launch and run a thriving online community but it is just not built for it. Managing multiple networks, channels, ever syncing communications, per-user billing and the search limit makes it a less obvious choice if scaling is your consideration.
I personally didn't use Slack because I find it difficult to keep up with the communities I belong to. Slack is a workplace communications app and will stay so. You can try out Mattermost or Quilt.co which are alternatives and in some cases better than Slack.
Like Slack, a Facebook Group comes with no initial investment from your end, all you need is a Facebook account. Facebook has recently launched a set of features targeted at Groups to help the community builders (they research trends more than anyone), with the mighty Facebook supporting it you can sit back and see the most wanted features rolled out to your community. This would have been the best option on the list IF there weren't boycotts, scandals, drama, highly-intrusive ads, disrespectful use of users' data, and overall hate that their actions generate.
I personally didn't use Fb groups because one is I didn't want my members to see an ad when they scroll through our community feed. Two is, I built our community around privacy and naturally, Facebook is a no go.
Discord is one of the best options on the list. It has chat, voices, channels and is quite complete. Discord's work all these years is amazing. They grew their platform by listening to the creators and consumers on it. Their business model is pocket friendly and works as an upgrade with perks, core features have always been free. Ages below 25 (aka the young generation) do not care what platform a community is running, give them value and that's all they care. The problem with Discord is they're meant for the gaming community and the whole environment is themed around gaming. Although they're working and constantly improving to become more open to the rest of the industries. It is still a work in progress.
I wanted a custom domain solution and the themes around games were negative to me.
Discourse is a more traditional and popular choice. Easy to setup, extensive resources, extension capabilities with plugins and its open-source nature, and it is free if you want to host it yourself. Its wide use helped many users become comfortable with its UI. Most product support and niche communities exist because of Discourse. Discourse may not be an option if you want to stay away from traditional forums. I would also say its UI is loaded. Some elements in the UI are unnecessary in my opinion.
I wanted more than just 'spinning off a Discourse instance', existing Discourse forums didn't seem alive to me for some reason. I do not mean the engagement rates. I mean the overall look and feel here. If you're looking for another option like Discourse then I suggest checking out NodeBB which is quite similar to Discourse.
When we say Forum you think of something that's running on Invision board or Xenforo. They both are long-standing and is recognized instantly. They're still a go-to for community builders because of how mature these products are. Both options are paid and IPB has a self-hosting license available. Each has a huge library of add-ons, themes and a community.
It wasn't the best option for me. My preferences were not a forum.
It gets personal here. I wanted to go with Flarum the most! It was clean, threads were replaced by tags, ultra-fast and super lightweight. This could even run on shared hosting (not yet). The only reason to not choose this was it didn't have a membership system. Flarum is also a new entrant in this list, it has bugs, stability issues, update failures, high technical knowledge... It wasn't ready for production yet.
Modern and dedicated community platforms you can explore include:
Habitate - has a free plan for you to test drive. It integrates well with your website and is best for having engaging discussions. Built by @deepak09 here, it has got polls, a trending feed, events and uses a 2 click navigation for the entire community.
Playgroup - all their plans support unlimited members. @benmann is building it for us, community builders. Public communities, private communities (whose post' URL can be shared outside if you want to), affordable and scalable pricing plan are some of its features.
Haaartland - You only pay a flat fee (in cents $0.xx while writing this) for active members only. A huge plus depending on some metrics, if you have one thousand members but only 10 login for the given month, all you to pay is $3 for that month. It'll be even more helpful if your community is small (like 20 = just $6/m as opposed to $20 and $30+ in others for almost the same features).
Crowdstack offers unlimited everything and starts at $49/m, you can finally take out Fear of Growing Too Big (FGTB). They come with better management, engaging elements, public/private subgroups, and inbuilt memberships.
Openland is lit. It is chat-based and you can start paid communities with them, Openland only takes a cut in your payouts for the platform they offer to you.
Spectrum.chat is a chat-based platform where you can start a community. Spectrum is owned by Github and is constantly improved by the team.
If you're looking for alternatives to Spectrum you can tryfave.com which is built on the same open-source software behind Spectrum with additional features like login for example and support from the team.
Group App from @Dornubari here is a Facebook + Slack hybrid dedicated for creators. It lets you create your community network, offer courses to your members, introduce a paywall and more. With Group App's discovery system you're more likely to be discovered by interested members. They even have a generous free plan (forever free plan I guess).
Community is for you if you're looking for SMS communities. It may also be the best for you if you have a huge following or highly engaged audience and you want to develop 1-on-1 relationships with them. SMS is personal. Celebs with millions of followers are posting "text my number" because of them.
Mighty Networks is a well-established platform. It is known for its powerful all-in-one software. It offers an intuitive community platform and built-in courses and is best for you if you're looking for branded mobile apps. MN has a forever free plan for you to checkout, all their plans include unlimited members as well. Personally, I didn't choose them as it seems more focused on mobile experience and branding, which wasn't my focus.
Tribe has got comprehensive social components that will help you build a highly gamified and engaging community for your members. Tribe suits best for product communities and private networks. It's a hybrid of social media and community. Given its widgets and API, it can be tightly integrated with your website and your product or service. Tribe also has a generous free plan capped at 500 members, you'll however have to upgrade to a paid plan to introduce a paywall.
I didn't choose Tribe because right after evaluating it and moving it to the top of my list I saw Circle.
May sound biased/give you a feeling that I'm affiliated with them but no. Circle was the best of all from what I have seen, I highly recommend them. When I was researching about them the good comments were like "What's this software, it's great I want one too" "This community UI is just peaceful". Bad ones were like "It lacks integrations" "Many features like polls, paid memberships, events are not there", which is forgivable for how new they are. It can't yet be compared with something as mighty as Mighty Networks.
Circle comes with a clean, well thought, no clutter user interface. Circle is asynchronous and has plans for making messages synchronous. It integrates well with Zapier for memberships, invites, comments and few actions. Their backend gives you full control over branding, domains, and CSS injection.
My experience so far is good with them. I'm glad I chose Circle over other options. A disadvantage is that they're new and don't have mobile apps yet, their iOS app is in the works though. The platform we have got for our community is clean and helps me offer the premium experience I intended. Now I just hope it stays this way as the product matures with new great features.
On a separate note, community builders are invited to join.coulf.com too, even if it is a directly competing idea. I struggled to get support from communities other than IH while building Coulf as it's seen as a competing product. So I'd like to support healthy competition and offer support as long as one is indie.