Some other open Slack alternatives I know about include:
Matrix[1]: maybe the most impressive. A chat _protocol_ with multiple server and client implementations, gateways to everything, and just out of beta.
Zulip[2] (as mentioned), Rocket.Chat[3], Mattermost[4]---Slack clones. open source, developement open to pull requests to various degrees, can pay to have hosted in cloud or get support.
Non-free or qualitatively different solutions include Discord, MS Teams, and the original IRC, I guess.
Apologies for any inaccuracies or omissions; I'm not an expert in the space.
Matrix[1]: maybe the most impressive. A chat _protocol_ with multiple server and client implementations, gateways to everything, and just out of beta.
Zulip[2] (as mentioned), Rocket.Chat[3], Mattermost[4]---Slack clones. open source, developement open to pull requests to various degrees, can pay to have hosted in cloud or get support.
Non-free or qualitatively different solutions include Discord, MS Teams, and the original IRC, I guess.
Apologies for any inaccuracies or omissions; I'm not an expert in the space.
[1] https://matrix.org/ [2] https://zulipchat.com/ [3] https://rocket.chat/ [4] https://mattermost.com/