Best open-source alternatives to Slack

Rocket.Chat is an open-source communications platform aimed at organizations that need secure, customizable collaboration tools. It supports team messaging and real-time conversations for internal coworkers as well as external communication with customers, citizens, or partner companies. The project is designed for self-hosted, cloud, and air-gapped deployments, making it suitable for regulated or high-security environments. It also includes security and governance features such as identity management, end-to-end encryption, and role- and attribute-based access control, while offering extensibility through a marketplace, Apps-Engine, and external integrations.

Mattermost is an open core collaboration platform designed for teams that want to self-host their communication and workflow stack. It provides a web-based interface and native clients for desktop and mobile, and is positioned for use cases such as DevSecOps, incident resolution, and IT service desk operations. The project is the primary source for core platform development and is implemented in Go and React. The software runs as a single Linux binary and depends on PostgreSQL for storage. The README emphasizes deployment flexibility, with installation paths for Docker, Ubuntu, tar archives, Kubernetes, and Helm. It also points users to product and developer documentation, and highlights a broad integration surface including webhooks, slash commands, drivers, web services, apps, and plugins.

Zulip is an open-source team chat platform designed to help organizations manage both real-time and asynchronous communication in a more structured way than traditional chat apps. Its signature feature is topic-based threading, which keeps conversations organized and makes it easier for distributed teams to follow multiple discussions without losing context. The project is aimed at teams, open-source communities, and other organizations that need productive group communication. The README highlights self-hosting on Ubuntu or Debian Linux, deployment via Docker, and hosted cloud options through Zulip Cloud. It also emphasizes a large contributor community and extensive documentation for both code and non-code contributions, suggesting a mature project with a broad user and developer ecosystem.

SimpleX Chat is a privacy-focused messaging platform built around the idea of avoiding user identifiers entirely. It is intended for people who want private conversations, metadata protection, and secure communication without relying on traditional account-based identity systems. The README describes mobile apps for Android and iOS as well as a terminal/CLI app for Linux, macOS, and Windows. Users can connect by sharing a link or scanning a QR code, then exchange messages, join groups, and make video calls. The project also highlights double ratchet end-to-end encryption, a directory service for user-created groups, and a community-oriented workflow for support, updates, and translation contributions.

Tinode is an instant messaging platform intended as a modern, open replacement for XMPP-style federated chat systems. It provides a backend written in Go and clients for web, Android, iOS, and a scriptable command-line interface. The project is aimed at teams or individuals who want a self-hostable messaging stack with mobile and browser access. The server supports messaging, calls, channels, access control, and multi-device synchronization, while also offering administration tooling and support for custom authentication backends. It can be deployed with different database backends and uses JSON/websocket or protobuf/gRPC transports. The README also points to public demo and sandbox services for trying the system before self-hosting.

Element is a Matrix client aimed at people who want to access Matrix chats through either a web browser or a desktop application. The README positions it as the main Element Web project in a larger monorepo, with related components and documentation links for installation, configuration, development, and translation. It can be used immediately through the hosted service at app.element.io, or self-hosted by following the installation documentation. The project also supports a desktop experience through an Electron wrapper, with pre-built downloads available from Element’s site or a path to build it from source. The README emphasizes browser compatibility, community triage, and a commercial licensing option alongside open-source licensing.

ejabberd is an open-source realtime communications platform built on Erlang/OTP. It is designed for administrators and organizations that need a scalable backend for messaging and related network services, and it bundles an XMPP server together with MQTT broker and SIP service functionality. The project can be installed from source, via operating-system packages, through Homebrew, or by using published Docker container images and release installers. Its README points users to the ejabberd documentation site for setup and administration guidance, and to separate developer documentation for contributors. The project also notes a commercial offering and support path through ProcessOne, including a Business Edition and cloud-hosted service.

The Lounge is a self-hosted web IRC client intended for people who want a persistent, browser-based way to stay connected to IRC networks. It is positioned as a modern alternative to traditional IRC clients, with features such as push notifications, link previews, and message markers to make IRC feel more contemporary. It is aimed at users who want their IRC presence to persist across devices and sessions. The README emphasizes that it stays connected while the user is offline, works across platforms wherever Node.js runs, and provides a responsive interface for desktop, smartphone, and tablet use. The project also documents a source-based installation path for development and links to a Docker-based deployment option for stable releases.

Colanode is an open-source collaboration workspace designed for self-hosting and local-first use. It targets individuals and teams that want chat, pages, databases, and file storage in a single platform while keeping control over their own data. The project emphasizes privacy, offline-friendly workflows, and the ability to connect to multiple servers from one client app. The system is built around a client app available on the web or desktop and a self-hosted server. Users choose a workspace after logging in, then collaborate through messages, page editing, and database updates. Data is written to a local SQLite database first and synchronized in the background, and the project uses CRDTs via Yjs to support concurrent edits on shared content.

Campfire is a web-based chat application designed for teams or groups that need a self-hosted messaging system. It provides familiar collaboration features such as rooms, direct messages, file attachments, search, mentions, and push notifications, along with access controls for rooms. The project is positioned as a single-machine deployment that bundles the web application, background jobs, caching, file serving, and SSL into one Docker image. The README also indicates that Campfire is intended to be administered locally by the operator, who creates an initial admin account on first launch. It is single-tenant, meaning separate customer groups would require separate instances rather than isolated workspaces within one deployment. The included API support suggests it can be extended for bot integrations and other external automation.