Medusa
medusajs.com
Open-source, headless commerce framework — build a fully custom online store backend as a free, self-hostable alternative to Shopify.
Why it is useful
Medusa provides the backend commerce logic (products, carts, orders, payments) as a Node.js framework rather than a locked storefront, so developers can build any frontend on top and self-host without Shopify's transaction fees and app-store markup. It's a better fit than Shopify for businesses that need deep customization or want to avoid platform lock-in.