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.

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.

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.

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.

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.

Ergo is a modern IRC server intended for operators who want a self-hosted chat network with built-in services and strong IRCv3 compatibility. It combines core IRC daemon behavior with account management, channel registration, host management, and bouncer-style features so users can reconnect from multiple clients and keep conversation history available. The project is aimed at administrators who want a configurable, reloadable server that is relatively easy to deploy and tune. It uses YAML for configuration, supports runtime rehashing of server settings and TLS certificates, and includes security-oriented features such as SASL, LDAP integration, bcrypt password storage, client certificates, and optional Tor hidden-service operation.

Openfire is an open-source XMPP server aimed at real-time collaboration and instant messaging. It is part of the Ignite Realtime community and is described as easy to set up and administer while emphasizing security and performance. The project is intended for organizations and developers who need a standards-based messaging backend built on the widely adopted XMPP protocol. The README highlights a Maven-based build, modular project structure, plugins, and platform-specific installers, along with instructions for launching the server from the command line after building. It also points to documentation, community support, bug tracking, nightly builds, and translations.

Buzz is a self-hostable workspace for teams that want humans and AI agents to operate in the same shared room. It combines chat-like collaboration with an event-log model, so messages, reactions, workflow steps, reviews, and git events all live in one signed stream. The project positions itself as a developer-focused alternative to stitching together chat, bots, CI dashboards, and release tools. It is designed for people who run projects, review code, manage releases, or delegate tasks to agents. The README describes channels, threads, DMs, canvases, media comments, search, audit trails, and agent-first workflows, along with a desktop app and CLI for local use. It also supports self-hosted relays and a hosted multi-community setup, with each community isolated by URL and identity model.

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.

Movim is a federated blogging and chat platform built around the XMPP protocol. It provides a web-based frontend for people who want to access XMPP services through a browser, combining publishing and messaging in one interface. The project is intended for users or communities running their own Movim instance as well as for people joining existing public instances. The README points to installation instructions in the repository and a wiki for deployment details, and it also includes a local container-based quick test setup using Podman and podman-compose.