Apart from already mentioned Mattermost[0] and rocket.chat[1], Matrix[2] and their main client Riot[3] is seriously worth checking out. It's a quite ambitious e

Apart from already mentioned Mattermost[0] and rocket.chat[1], Matrix[2] and their main client Riot[3] is seriously worth checking out. It's a quite ambitious effort for decentralized, federated IM / group communication. There's still some work left on the protocol in terms of federated identity, which should not be necessary of you're looking to replace Slack. I haven't spent significant time with it in the past year or so, but from my understanding it's starting to become production ready. IMO there's value in pushing for decentralized, federated alternatives. Would be interested to hear from people who have used it in anger. [0]: https://mattermost.com/ https://mattermost.com/ [1]: https://rocket.chat/ https://rocket.chat/ [2]: https://matrix.org/ https://matrix.org/ [3]: https://about.riot.im/ https://about.riot.im/