Synapse

Synapse is the reference server for the Matrix protocol, giving you self-hosted encrypted messaging with no vendor lock-in. Matrix is to chat what email is to messaging: decentralized, federated, and interoperable. Your server talks to other Matrix servers, so your users can chat with anyone on the network.

Fully free under Apache 2.0. You get end-to-end encrypted messaging, voice/video calls (via Jitsi or built-in), rooms, spaces (like Discord servers), bots, bridges to other platforms (Slack, Discord, IRC, Telegram), and full admin control. Element is the most popular client app.

The catch: Synapse is a resource hog. Even a small server with a few hundred users needs 2-4GB RAM, and it grows fast. Federation, the feature that makes Matrix special, also makes it slower and harder to moderate. If you just need team chat and don't care about federation, Rocket.Chat or Zulip are lighter. And the initial setup (Docker Compose, reverse proxy, DNS, federation verification) takes a solid afternoon.