Predominantly JS, HTML, CSS with server-user communication in WebSockets (though WebRTC support could be nice in future). If you’re good with any of these and want to help the team / ask anything, message me here, Skype drawplanet, or email firstname.lastname@example.org. Will be more than happy to answer any of your questions. And if you’re crtical of anything, nothing’s set in stone. It’s very much a by the community, for the community type of thing.
Here’s the interface (though an old version as we’re constantly improving; mobile has a separate UI).
Did I mention it’s not Flash?