Code Playground

http://www.smashingapps.com/2014/05/19/10-code-playgrounds-for-developers.html

jsFiddle
Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.

LiveGap Editor
Free Online Html Editor with Syntax highlighting, live preview, code folding, fullscreen mode, themes, matching tags, auto completion, finding tags, frameWork and closing tags.

Codepen
CodePen is an HTML, CSS, and JavaScript code editor in your browser with instant previews of the code you see and write.

Cssdesk

Google Code Playground
The AJAX Code Playground is an educational tool to show code examples for various Google Javascript APIs.

jsbin
HTML, CSS, JavaScript playground that you can host on your server.

Editr

Ideone
Ideone is something more than a pastebin; it’s an online compiler and debugging tool which allows to compile and run code online in more than 40 programming languages.

Sqlfiddle
Application for testing and sharing SQL queries.

Chopapp
A little app from ZURB that lets people slice up bad code and share their feedback to help put it back together.

Gistboxapp
GistBox is the best interface to GitHub Gists. Organize your snippets with labels. Edit your code. Search by description. All in one speedy app.

D3-Playground