StackBlitz

Stay in the flow with instant dev experiences. No more hours stashing/pulling/installing locally — just click, and start coding.

https://stackblitz.com/

StackBlitz is great since it supports backend you can use Express/NestJS/Koa etc.

Of course, it supports FE stack

CodePen

CodePen is a social development environment for front-end designers and developers.

https://codepen.io/

I think CodePen is awesome since we can find out really interesting stuff (3D stuff by threejs/amazing css animation/super cool UIs etc)

JSFiddle

JSFiddle is very simple but still really functional. If you like a simple UI, JSFiddle will be good for you.

https://jsfiddle.net/

JS Bin

JS Bin is one of the simplest javascript playgrounds.

https://jsbin.com/

PlayCode

PlayCode is a website where you can type in code and it immediately shows you what you programmed in a little extra tab. All you need to do is select a programming language or a framework and you can start without needing to set up anything.

https://playcode.io/

code.sololearn

code.sololearn.com supports some languages. If you want to use a few language with the same UI. code.sololearn.com would be a good choice.

https://code.sololearn.com/

CodeSandbox

CodeSandbox is an online code editor and prototyping tool that makes creating and sharing web apps faster.

CodeSandbox is great since it supports FE and BE(node/nestjs) and it has a few unique templates.
CodeSandbox is my favorite. I always use React Typescript template.

https://codesandbox.io/