Real estate site Trulia has created crime maps which let you see how violent different neighborhoods are based on data from CrimeReports.com, EveryBlock.com and SpotCrime.com. Below is Philadelphia:

by comparison, here is a similar map produced by the DC metropolitan police department, except this one is a visual piece of crap:

Below is one of the same area produced by spotcrime.com, which I like because it uses cute little little fists, burglars, and spray paint icons:

and for most disturbing map, we have this one from crimereports.com, which in addition to detailed crime information, includes pop-up pictures of registered sex offenders in your neighborhood.