He has commented below that the 'front end is GWT, back end is Java servlets. Database was originally MySQL but switched to Prevayler for performance reasons.'

I have built a similar tool that is also an HTML5 / Canvas collaborative drawing app which uses Google Drive for storage and has no server code beyond that. It works offline (and syncs up with collaborators when you come online) and runs in Chrome, Firefox, Safari, iOS, and Android and supports touch. http://thesavior.github.io/draw/