I tested about 5 code playgrounds today:

1. http://dabblet.com/
2. http://codepen.io/
3. http://jsfiddle.net
4. http://jsbin.com
5. http://cssdeck.com – winner

CSS Deck was the winner for various reasons: when the code is embedded in another site, you can view multiple panes of code at once and each pane has line numbers. The others do some great things but not both of these.

Example from CSS Deck from another developer:

If however, you need to store/share code other than JS/HTML/CSS, then GitHub seems the best. See below:

Here's an easy/effective way of moving all JS to the bottom of all pages in WordPress: