- https://codepen.io: My favorite for low overhead/commitment rapid client side experimentation. Split screen views, live reload, sharable, no account necessary, etc.
- https://glitch.com: My all time favorite for full app/website experimentation and prototyping (or even hosting). Feature rich editor with multi-file and console support. Like a computer in the browser. Does serverside Nodejs and gives you a full virtual machine in the browser environment. Really great community, built for remixing/sharing, easy to go from prototype to production-y app effortlessly. Learner/beginner friendly.
- https://observablehq.com: Reactive notebook environment (from creators of D3, Backbone.js, etc.). Has some very powerful features but the environment is slighly idiosyncratic (different than normal coding in an editor). Really well suited for remixing, prototyping, and sequential things.
- https://codesandbox.io/: IDE in the browser. targeted at rich client side applications (React, Vue, Svelte, etc.) and more development/production-y things.
- https://repl.it/: Multi language support.
- https://runkit.com: Similar notebook-like environment. targeted at Nodejs and more server side development?
- https://jsfiddle.net/: older but similar to Codepen. Similar features but maybe less actively maintained and less commercial/company support behind it.
- https://jsbin.com/: Similar to JS Fiddle but not actively maintained?