Slack is described as «unified communication platform built with an optimized workflow to keep modern teams focused and productive. A familiar chat interface facilitates discussions while advanced features are easily exposed based on context» and is a leading Group Chat app in the social & communications category. There are more than 100 alternatives to Slack for a variety of platforms, including Android, iPhone, Windows, iPad and Mac. The best alternative is Mattermost, which is both free and Open Source. Other great apps like Slack are Discord, Element, Zulip and Rocket.Chat.Slack alternatives are mainly Group Chat Apps but may also be Video Chat Apps or Encrypted Chat Apps. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Slack.