CodePen is a great way to hack out an idea and instantly see results. No need to setup a dev environment. And one of my favorite features of CodePen in particular is the SCSS-to-CSS preview. I can write in SCSS and if I want to see the CSS compiled version, I just click an icon to toggle.
JSBin and JSFiddle, where I also have code stashed, are solid alternatives to CodePen. Each have their strengths and weaknesses. For example, I like that JSBin has a Console pane for debugging JS. And I like JSFiddle's 4-pane layout. But I'm very much a fan of CodePen's slick design and its social features.
Here are some of my pens from CodePen...
See the Pen Browser Title Bars in CSS by Glenn Cueto (@glnster) on CodePen.
See the Pen Vertical Center Hacks by Glenn Cueto (@glnster) on CodePen.
See the Pen Grilled cheese sandwich menu icon by Glenn Cueto (@glnster) on CodePen.
See the Pen html/css-only slider by Glenn Cueto (@glnster) on CodePen.