When I'm exploring a neighborhood, I often find myself, with no knowledge of the area, searching in vain for the major commercial strips in an area. I might walk the length of brooklyn's 7th ave, for instance, only to discover that 5th and 8th are lined with goodies.
These zoning maps, from the nyc.gov website, show which streets are zoned for commercial use, and thus, which streets are worthy of investigation. Shaded boxes on the map indicates commercial zoning. Enjoy.