I'm a big fan of Superpowers (although I'm likely the only one): http://superpowers-html5.com
It's a great collaborative HTML5 engine that actually runs in the browser. If you set up hosting for it you can get google-docs style collaboration, which is not great for proper software engineering but it's awesome for game jams!
It definitely has a few bugs and quirks though but what engine doesn't honestly? That being said I'm happy to help people get started with SP if they're interested.