Rocket.Chat

open-source Slack alternative with omnichannel support

Overview
Company	Rocket.Chat
URL	rocket.chat
Pricing	free— Commercial/Government/Defense tiers are custom-priced (free self-hosted Community edition available)
License	open
Hosting	hybrid

MIT-licensed, self-hostable team chat with channels, voice/video, and broad integration support. Its omnichannel inbox unifies live chat, email, WhatsApp, and Telegram into one agent queue -- useful if you also want to replace a separate customer-support tool, not just internal chat.

Usage Notes

Has the most GitHub momentum of the self-hosted Slack alternatives (~45k stars). The free Community edition covers most day-to-day needs; exceeding Starter-plan limits blocks adding new users but does not lock you out of existing data.