CodePen

Open source social development environment for coders

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Hunter's comment

CodePen is a free open source code editor where front-end designers and developers can experiment with code snippets, build and deploy websites, collaborate with other coders to develop applications and make reference to code created by others.

Check out this introduction video to CodePen:

Here are the key reasons why CodePen is a useful resource for coders:

Write and share front-end code: HTML, CSS, JavaScript
See in real-time what your code does
Learn new stuff by experimenting & collaborating
Build reduced test cases & seek help from the community for tricky code
Build components to re-use
Create a code repository of the great stuff that you build; that you can share with others

I used CodePen when I was learning to code with FreeCodeCamp.

Images are from CodePen