Mattermost
Team chatMattermost is an open source platform for secure collaboration across the entire software development lifecycle..
39k Custom / see repo Active
Replaces Slack 
Rocket.Chat
Replaces SlackThe Secure CommsOS™ for mission-critical operations
Stars46,305
LicenseCustom / see repo
Last pushOct 21, 2026
Active
Forks14,019
Open issues + PRs4,205
LanguageTypeScript
GitHub topics:chatcollaborationfossfreehacktoberfestjavascriptmeteormit
Source: https://api.github.com/repos/RocketChat/Rocket.Chat, fetched Oct 21, 2026. "Active" means a push within 30 days of that fetch. Maintainer? Claim or get featured.
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