A few open source options (some with hosted plans) - Zulip - https://zulip.com https://zulip.com - Mattermost - https://mattermost.com https://mattermost.com

A few open source options (some with hosted plans) - Zulip - https://zulip.com https://zulip.com - Mattermost - https://mattermost.com https://mattermost.com - Rocket chat - https://rocket.chat https://rocket.chat - Matrix - https://matrix.org https://matrix.org