Tinode is an open source instant messaging platform with a Go backend, web/mobile clients, JSON WebSocket and gRPC APIs, and support for one-to-one and group chat.

13.5k stars2.1k forksGPL-3.0last commit Actively maintained
Revolt is a hosted community and team chat platform providing servers, channels, direct messages, voice chat, bots and integrations. It delivers real-time text and voice communication similar to Discord, with a cloud-hosted option and extensible bot support.
Tinode is an open source instant messaging platform with a Go backend, web/mobile clients, JSON WebSocket and gRPC APIs, and support for one-to-one and group chat.

13.5k stars2.1k forksGPL-3.0last commit Actively maintained
Tailchat is an open source team chat and collaboration platform with a strong plugin system, bot integrations, multi-group workspaces, and RBAC permissions.

3.6k stars397 forksApache-2.0last commit Actively maintained
Stoat is an open-source, self-hostable chat platform for communities and teams, offering text channels, permissions, moderation tools, and a web app.

3.3k stars383 forkslast commit Actively maintained
Every option on this page is open source and free to run on your own hardware, so you own the data and there is no subscription to cancel. 3 of 3 shipped a commit in the last six months. Licences in this list: GPL-3.0, Apache-2.0. In exchange you take on hosting, backups and updates yourself.
Browse everything in Team Chat & Messaging.