Sketch lets you draw on maps and have fun doing it. It’s based on Polymaps, and has a bunch of superpowers like touch compatibility, streaming changes, different inks, text mode, and lots else that’s in the code but not in this thrown-together demo on top of World Glass.
This is something of a MapBox leak: sketch is a component we’re using in a big project that goes way farther. This is the tip of the iceberg - the fully-open-source, in the public tip.
Sketch speaks GeoJSON fluently and exclusively, so you can hook up your drawings to your favorite serious map-authoring kit, like TileMill. It draws with SVG, and is super simple to add to Polymaps, just
Oh, and since this is kind of crazy stuff, it doesn’t work with IE. Sorry!