I'd also like to be able to see\toggle the skins like you can in the bike demo.
and why not also include a link to download the javascript source?! No need to hide the unminified versions in the html

<!-- Un-minified versions of the above
<script type="text/javascript" src="box2dweb.js"></script>
<script type="text/javascript" src="loadrube.js"></script>
<script type="text/javascript" src="testbed.js"></script>
-->
When you export the json from the editor, it might also be nice to autogenerate the loadrube.js file too and drop it in the same directory. When I first looked at the exported json I thought "Great! now how am I going to parse this sucker...?!" but obviously that's already taken care of within loadrube.js.