There are a couple of new very cool web apps that use the ability to pre-fill a new Pen with provided code:

Type Scale

TypeScale

Type Scale gives you a visual model of what type sized in different proportions looks like. For instance, the Golden Ratio is 1.618. So if the base is 1em, a size up is 1.618, then 2.618, then 4.236 etc. I'm partial to the Major Third myself.

Get some text looking good then "Edit on CodePen" to continue playing.

Polynode

Polynode is a web app that takes 3D model files (.STL) and converts them into fully interactable SVG right in the browser. Get it looking just right then export it to CodePen to save it or continue playing.

polynode

Made from the folks at Xpressive Team who do some amazing work with 3D in the browser. They said they took some inspiration from TriDiv which perhaps you've seen before.