The Gotham Gazette's Wonkster reports the request from the site for crime data were denied by the city police force. NYC's ComStat (typo corrected per comment) program is widely praised, but the data unlike that from Chicago is not provided at the block level. The next step? A Freedom of Information Act request.

That post linked me to yet another interview with Adrian Holovaty, the fellow behind EveryBlock. Here he mentions one the open source mapping tools:

We use an open-source library called Mapnik to render the maps, so that library does the heavy lifting for us. Paul is also working on a how-to article, in the spirit of giving back to the open-source community, that explains how to use Mapnik.

I know of Mapnik, but this forced me to finally confirm what I'd always suspected: the fellow behind it, Artem, was at Cadcorp with me back in the day. (Interview with Artem at Nestoria here.) The world is small my friend!

Also of note, an article by Paul Smith, EveryBlock's mapping guy, that explains the why and how of the solution.