You can find a map of the NYC subway system by going to MTA.info and clicking on the Maps tab at the top of the page. From there, you have the ability to download subway maps that include the entire subway system, as well as maps by neighborhood.
If you do not want to download the map, you still have the ability to view them by clicking on the link of the map you would like to view. Other subway maps available include maps for late night service, service guides, weekend subway maps and individual subway line maps. You can also utilize the MTA's mobile application, which allows you to have access to the NYC subway maps through your mobile phone.
MTA.info also has maps of the NYC bus transportation line, as well as the railways that the MTA serves. Maps available for the bus lines include the Bronx, Brooklyn, Manhattan, Queens and Staten Island. The railway maps available include the Long Island Railroad, Metro North Railroad and the Staten Island Railway.
The MTA also provides residents and visitors with maps of the New York City area tunnels and bridges. The MTA notes that the maps should not be used for anything other than personal use.