NodeBB

A forum platform built on Node.js and WebSockets instead of Discourse's Rails stack.

Best for: Teams who want a modern, real-time-feeling forum without taking on Discourse's Rails/Ruby footprint.
Skip if: You need Discourse's larger plugin ecosystem and maturity, or you'd rather not run MongoDB/Redis yourself.

NodeBB is an open-source (GPL-3.0) discussion platform written in Node.js, using Socket.IO for real-time topic updates and notifications instead of page reloads. It supports MongoDB, Redis, or PostgreSQL as its datastore, ships official Docker images, and is themeable/extensible through a plugin system. Self-hosting is free; NodeBB Inc. also sells managed hosting plans.

Pros

Pure Node.js stack - no Ruby/Rails ops burden, WebSocket-driven UI feels instant
Flexible datastore choice (MongoDB, Redis, or Postgres) plus official Docker images
Actively maintained with a plugin and theme ecosystem

Cons

Smaller community and plugin ecosystem than Discourse
GPL-3.0 licensing means a closed-source derivative needs a commercial sublicense
Hosted plans range from $20/mo (Starter) up to $750/mo (City) depending on traffic tier