Slack→3 open-source options
Open-source Slack alternatives
Commercial team messaging and collaboration platform. Below are curated open-source options listed on OpenElse, with notes on when each is a better fit.
Official site: slack.com
Mattermost
Self-hosted team messaging focused on secure collaboration for engineering teams.
Rocket.Chat
Open-source team chat and omnichannel platform with channels, video, and customer messaging.
Zulip
Topic-threaded open-source team chat built for asynchronous, low-noise discussion.