Remix is an open-source web framework, copyrighted by Shopify, Inc. and published under the MIT license. A web framework is a set of libraries and conventions that handles routing, server code, data loading and rendering for a web application. Remix has two generations. Remix v2 is a full-stack framework for React, built on top of React Router; its documentation now says that the latest version of Remix is React Router v7.
In Remix v2 each route module exports a loader, which loads data on the server, an action, which handles form submissions, and a component. A compiler based on Vite builds a server handler and a browser bundle split by route. The handler uses the Web Fetch API, so an app runs on Node.js servers, Cloudflare Workers or Deno Deploy. Remix 3, the version on remix.run, is a new stack built on Web APIs: a server runtime, routing, authentication, sessions, database integrations, a UI framework and assets that compile on request, with no build step.
Both generations are free; Remix 3 is developed in the remix-run/remix repository on GitHub.