Mattermost

open-source Slack alternative with self-hosted free tier

Overview
Company	Mattermost
URL	mattermost.com
Pricing	free— Professional $10/user/mo; Enterprise custom pricing (self-hosted free tier available)
License	open
Hosting	hybrid

Self-hostable, MIT-licensed team chat closely modeled on Slack's channel-based UX -- includes a Slack export/import path to ease migration. Free self-hosted tier runs indefinitely with no per-seat licensing cost; paid tiers add SSO, compliance exports, and HIPAA/FedRAMP-aligned deployment options.

Usage Notes

Fewer native integrations than Slack out of the box -- most third-party connections go through incoming/outgoing webhooks rather than a marketplace, so budget some setup time. Good fit for teams that want a near drop-in Slack replacement and value Go-based, ops-friendly self-hosting.