Repl.it
Online REPL for 15+ languages
...The project’s core idea is instant, zero-setup programming: open a page, pick a language, type, and run—no local installs or environment wrangling. It combines an in-browser editor with a runnable backend or sandbox so code can execute safely and return output in seconds. Sharing and collaboration are first-class: code can be saved, forked, and embedded, which makes it useful for tutorials, classrooms, and quick demos. The architecture leans on simple web technologies so the learning curve stays low for educators and new programmers. ...