This is dedicated to Mootools right now but it shows huge promise. It has four panes, HTML, CSS, JS and the result. Unlike in JSBin, the result isn’t accessible outside of the editing space so the Ajax mimicry isn’t quite as advanced yet. MooShell though is pretty cool and I’d love to see it put on GitHub, I’d love to help add features. You really need to try MooShell sometime, with your favorite Mootools code.
Share similar testing utilities you’ve seen or even made. I actually one similar to these a year ago for personal use.