Ryver is a U.S.-based team collaboration platform that combines chat, topics, and task management in one app, replacing a Slack and Trello combination. Starter starts at $69/month with unlimited messaging, task boards, and voice/video calls.
Zulip is an open-source team chat platform created in 2012 and released under Apache 2.0 in 2015, maintained by Kandra Labs and headquartered in San Francisco, United States. Its unique Stream+Topic threaded model makes async communication efficient and orderly, with unlimited users and self-hosting on the free plan.
Rocket.Chat is an open-source team communication platform initiated in 2015, offering free self-hosting, end-to-end encryption, and Matrix federation, officially serving 12M+ users and 100,000+ organizations - a Slack alternative for organizations that prioritize data sovereignty and compliance.
Mattermost is an open-source team collaboration platform founded in 2014 in Palo Alto, California, supporting private deployment with unlimited users. Adopted by NATO and the US Department of Defense, its free edition includes 10GB file storage for data-security and compliance scenarios.