So, it's years later now. Services have been started, bought, sold, etc since I first asked this question. It seems that the dominant service being used these days (at least for testing/sharing html, js, and css is jsfiddle.
If you're looking for a replacement to Etherpad (which was bought and shut down by Google) then you can try typewith.me, though I've found the service to be buggy every time Chrome gets an update.