This is a late introduction of dojox.sketch, which was contributed to dojo foundation by TeamPatent.

dojox.sketch allows users to place/manipulate shapes on top of a background image, zoom/pan, and (unlike other open-source Javascript drawing tools) supports undo/redo. It's designed for collaboration, encoding changes as diffs to the SVG document (the diffs can be transmitted to collaborators). It's cross-browser, transposing SVG to VML and vice versa on the frontend.

It was first implemented by Tom for dojo 0.4 under a contract with TeamPatent and later I ported it to dojo trunk. The code is experimental (alpha quality) and you can try it out here. Placing shapes atop a bitmap makes it similar in concept to the Adobe Acrobat drawing markup toolbar though, at this point, it only supports callouts. dojox.sketch could use easier placement/manipulation of shapes (e.g. group selection and drag/drop off toolbar) and more shapes.

A couple of days after it was contributed, a Comet Daily contributor added Comet to dojox.sketch. If your project could use Sketch, please consider extending it and contributing back.

On a side note, TeamPatent is also looking to hire a full or part-time software engineer with backend expertise and an interest in collaborative systems (our prototype currently uses Python and PostgreSQL). TeamPatent is funded by a grant from the National Science Foundation which restricts the work to US residents.