OwnBase

OwnBase is open-source team chat that you host. Run the server on a VPS, or host a workspace from the desktop app on your own PC, the way you would open a Minecraft world to LAN. Teammates on the same Wi-Fi see it in the app’s Join screen, and every message, file and account stays in one folder on the host’s machine.

It has channels, threads, DMs, search, and WebRTC calls with screen share, kept in sync over WebSockets from a per-workspace event log. A Slack-compatible API means most existing Slack integrations work once you change the URL, and an Enterprise edition adds single sign-on and a control plane for hosting many workspaces.