Zulip

It is a team chat application with topic-based threading that organizes conversations to support asynchronous communication for distributed teams. It is for developers, IT teams, and businesses including distributed organizations, educational and research institutions, and open-source communities. It is delivered as a cloud-hosted SaaS with a free tier and enterprise plans and as self-hosted open-source software with desktop and mobile apps, integrations, and an API.

Key features

• Topic-based threaded conversations
• Inbox for unread messages
• Desktop and mobile apps
• Integrations
• API access
• Self-hosting option
• Cloud hosting