Let's Chat is a persistent messaging application that runs on Node.js and MongoDB. It's designed to be easily deployable and fits well with small, intimate teams.
License: Open Source
Slack is a comprehensive team collaboration tool with robust features like video calling and extensive integrations, making it ideal for larger organizations and remote teams. Let's Chat, on the other hand, is an open-source solution that is lightweight and customizable, making it a good fit for small teams or businesses that prefer self-hosted options.