Rocket.Chat

Rocket.Chat is a powerful open source web chat server, similar in function to Slack, enabling team collaborations and communications. Rocket.Chat is scalable from the smallest two-persons project team, to enterprise with tens of thousands of users.

Rocket.Chat has a modern chat interface that supports emojis, reactions, link previews, voice, photos, and video.  Major features of Rocket.Chat include documents sharing, voice messages, as well as video and audio conferencing; private groups and off the record messages allow encrypted private conversations. Public channels enable collaboration across departments and with partners.