A list of online coding sandbox/playground utilities.
Mostly JS and Shaders. All allow quick coding and running in the browser without even signing up.
JSFiddle - Javascript Code playground
Codepen - Similar to JSFiddle, but not the same
Livegap - HTML & JS editor
WasmFiddle - Online WebAssembly compiler
WebAssembly Explorer - Compile C and C++ to wasm
Dittytoy - Javascript Music Creation
Shadertoy - GLSL Shaders coded in the browser.
VertexShaderArt - Like Shadertoy, but with vertex shaders and music
GLSL sandbox - Similar to the above
WGSL sandbox - this one's the future, uses WGSL and webGPU
WebGPULab - another great place to try out the latest 3D browser api
CodeSandBox - Enterprise level browser dev environment
Phoenix - Brackets-based editor and complete coding IDE
This post is a work in progress, let me know if you use anything no listed here.