LAPD Crime map

http://lapdcrimemaps.org/

here's one for the city of SF

http://www.sfgov.org/site/police_index.asp?id=23813

other cites:
chicago - www.chicagocrime.org
portland - www.gis.ci.portland.or.us/maps/police
houston - houstoncrimemaps.com
minneapolis - www.ci.minneapolis.mn.us/police/crime-statistics/codefor
Berkeley, CA - http://www.beastblog.com/2005/08/02/crimes-in-berkeley-where/
NYC - homicide map - http://www.nytimes.com/packages/html/nyregion/20060428_HOMICIDE_MAP.html