Best open-source alternatives to Slack

Rocket.Chat is an open-source communications platform aimed at organizations that need secure internal and external collaboration. The README positions it as a team messaging solution for companies, public-sector users, and regulated environments where privacy, compliance, and control over data are important. It supports multiple deployment models, including self-hosted, cloud, and air-gapped setups, and highlights workspace management, federation, identity management, encryption, and role- and attribute-based access control. The project also includes desktop and mobile apps, an apps marketplace and Apps-Engine for extensions, and API and developer documentation for integration and customization.

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 application built around topic-based threading. It is aimed at organizations and communities that want structured conversations that work well for both real-time discussion and asynchronous collaboration. The README highlights its use by large companies, open-source projects, and many other organizations, emphasizing productivity for remote work. The project can be explored through Zulip’s public community chat without creating an account, or deployed self-hosted on Ubuntu or Debian Linux, via Docker, or through prebuilt marketplace images. It also offers a hosted cloud option for teams that do not want to manage their own server. The README points to extensive contributor documentation and describes the project as actively maintained by a large distributed open-source community.

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.