StackBlitz

StackBlitz runs Node.js directly in the browser using WebContainers, so a full JavaScript project can start in seconds without a server. You open a link, and the editor, terminal and preview are all there.

It is popular for sharing reproducible examples in docs and bug reports. The same technology powers Bolt.new, which adds an AI agent on top.

It works best for frontend and Node projects. Anything that needs a native database or non-JavaScript runtime is outside its comfort zone.