Recently on Hackernews I found an interesting link. It pointed to RubyFiddle. The site is really nice, but my favourite bit was the editor.
By then I had already guessed they hadn't done it themselves so I investigated (looked in the source..) and found they're using CodeMirror.
I had heared about CodeMirror before but I never really liked online IDEs. But this editor really caught my eye so I decided to try the most basic setup.
That is all that is needed. Thanks to Marijn Haverbeke for such a great online editor.
Below is a demo of it: