Last time I looked, Riot, Mattermost, Friends, and Rocket were good options. This post compares five alternatives: https://blog.okturtles.org/2015/11/five-open-

Last time I looked, Riot, Mattermost, Friends, and Rocket were good options. This post compares five alternatives: https://blog.okturtles.org/2015/11/five-open-source-slack-alternatives/ https://blog.okturtles.org/2015/11/five-open-source-slack-al... (Disclosure: I work with okTurtles)