Collaboration in the enterprise has been historically seen as a document portal where people can collaborate. Microsoft Sharepoint has been the leading solution of this market. With the rise of chat-based collaboration like Slack, a new kind of solution emerged. Microsoft Teams is one of the fastest rising solution in the closed-source space.
Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.
Element is a free and open-source software instant messaging client implementing the Matrix protocol. Element supports end-to-end encryption, groups and sharing of files between users. It is available as a web application, as desktop apps for all major operating systems and as a mobile app for Android and iOS.
Rocket.chat is an open source team chat platform.