Synapse

Synapse is the reference Matrix homeserver, now maintained by Element. Matrix is to chat what email is to mail: federated and interoperable, so your server talks to every other Matrix server and nobody is locked inside one company's app. The core is free under AGPL v3, with encrypted rooms, spaces, voice and video, bots, and bridges into Slack, Discord, IRC and Telegram.

Running it is real work. Python and Rust over Postgres, behind a reverse proxy, with DNS and federation verification to get right on day one. A few hundred users wants 4GB of RAM and climbs from there, and media storage grows without asking. Federation is the point of Matrix and also what makes moderation and performance harder.

Solo: skip it unless running infrastructure is the fun part. Small teams: viable when someone actually wants the ops job, and Rocket.Chat or Zulip are lighter when nobody does. Medium teams: bridges and federation start earning their keep, so budget dedicated ops time. Large teams: price Element's hosted tiers before committing an engineer to this.

The catch is subtler than the sales page suggests. Synapse relicensed from Apache 2.0 to AGPL v3 under Element, which also sells a commercial license. OIDC and SAML single sign-on both work on the free build, so do not let the pricing table talk you out of self-hosting. What Element actually gates is the packaged enterprise layer: LDAP directory sync, SCIM provisioning, auditing and retention tooling, and a separate homeserver called Synapse Pro. Community is pitched at 1 to 100 users and non-commercial use, with no support at all unless you subscribe.