Here are some interactive playgrounds I've found (or built) on the internet:
- CSS/HTML/JS: CodePen by Chris Coyier, Alex Vasquez, and team
- CSS/HTML/JS: JSFiddle by Oskar Krawczyk and Piotr Zalewa
- CSS/HTML/JS: flems by Rasmus Porsager (saves all state in the URL)
- DNS: Mess With DNS by Julia Evans and Marie Flanagan
- DNS: DNS lookup tool by Julia Evans
- nginx: nginx playground by Julia Evans
- varnish: fastly fiddle from fastly
- SQL: sql playground by Julia Evans
- SQLite: sqlime by Anton Zhiyanov (lets you load arbitrary SQLite databases)
- Postgres: postgres playground from Crunchydata (runs postgres in the browser!)
- regular expressions:
- Git: oh my git by blinry and bleeptrack
- Arduino / IoT: wokwi from CodeMagic
- compilers: godbolt compiler explorer by Matt Godbolt
- .NET bytecode: SharpLab by Andrey Shchekin
- Shaders: shadertoy by Inigo Quilez and Pol Jeremias
- ASN.1 certificates: ASN.1 JavaScript debugger by Lapo Luchini
- Unicode: Unicode analyzer from fontspace
- Unicode: What unicode character is this? from babelstone
- SVG: sssvg (interactive SVG reference) from fffuel (lots of other cool tools there)
- Floating point: Float Exposed by Bartosz Ciechanowski
- CBOR: CBOR playground
- cron: crontab guru from cronitor
I would like to have this truly universal playground added to the list: Codapi.org. It covers programming languages, query languages, command-line tools, and probably anything you can run in a container. And it's open source.
A nice addition is the Try X in Y minutes series, based on the famous "Learn X in Y minutes" but featuring executable code snippets.
If this sounds like an ad, I apologize. I am only a happy Codapi user. (And I know the author. And I provided the Try Go in Y minutes page. But that's all, really.)