Are you trying to use Subway Trains, Busses or Buggies to get around town? Don't know your A Train from a B Train? http://www.hopstop.com/ is a very cool transit mapper, for New York, Boston, Washington DC and San Francisco.
Another Subway mapper for NYC is "onNY Turf". Going on an interview in an unfamiliar neighborhood? This is pretty nice, you just enter an address in and OnNY Turf will show you which subway stations are nearby. http://www.onnyturf.com/subway/
Just looking for cross streets without the maps? Click over to the NYC Address Locator