Open source alternatives to Slack

By GitHub stars it is Rocket.Chat, with 46,503 stars and a commit on 13 Nov. Choose Rocket.Chat when your chat history must stay on your own servers or an air-gapped network, with no per-seat bill for small teams. Next come Mattermost (39,606 stars) and Buzz (37,297 stars).

Yes. The code of all 12 is public under MIT, AGPL 3.0, Apache 2.0 or GPL 3.0, so you can use them with no licence fee. Running one yourself costs the server and your time to look after it, and some makers also sell a hosted plan.

Start from the job you use Slack for. Choose Buzz when you want to host it yourself and keep your data on your own server. Choose Mattermost when chat must stay on servers you control, even air-gapped ones, and the open source edition carries no per-user fee.

11 of the 12: Rocket.Chat, Mattermost, Buzz, Fluxer, Colanode, Campfire, Cinny, Stoat and 3 more. Each tool's page says what it needs to run.

11 of the 12 we track had a commit in the last 30 days, among them Rocket.Chat, Mattermost, Buzz and Fluxer. The quietest is Colanode, last changed on 3 Apr. A recent commit shows the project is kept up, not that it will stay that way.

AGPL 3.0 (7), MIT (2), Apache 2.0 (2), GPL 3.0 (1). MIT, BSD and Apache let you change and reuse the code freely. GPL and AGPL ask you to share your changes under the same licence when you give the software to others, and AGPL also counts letting people use it over a network.

The hosting. Slack's maker runs the servers for you; with an open source tool the updates, backups, security fixes and support are yours, unless you take the maker's hosted plan. Features differ too, so check what each one imports and exports before you move your data.