iOS

• 
  iPhone

  Google Maps

  get itFree

  Google Maps is back! Google's reborn iOS navigation app includes turn-by-turn navigation, transit routing, and, of course, Street View, traffic, and tons of point-of-interest data. Welcome back, old friend.
• 
  Universal (iPhone + iPad)

  HopStop Transit Directions for iPad

  get itFree

  Available for both the iPad and the iPhone, HopStop provides detailed mass-transit directions for most major U.S. and Canadian cities, plus a handful of European destinations. It also includes good walking and biking directions, along with schedules and maps for major transit providers.
• 
  iPhone

  Roadify

  get itFree

  This free app has public transportation info for most major U.S. cities, along with parking info for those who use their own wheels. You can also report issues with transit lines, and take advantage of its Twitter integration to see what other riders are discussing -- or share your own complaints about that bus that's 20 minutes late.
• 
  iPhone

  Scout by Telenav

  get itFree

  Scout is a robust GPS app with offline maps, turn-by-turn navigation (free for the first year, $9.99 per year after), extensive point-of-interest data, good traffic info, and flexible routing tools (including the ability to reroute based on traffic conditions).
• 
  iPhone

  Lumatic City Maps

  get itFree

  If you'd prefer to walk it, Lumatic is the best way to find your way around about two dozen U.S. cities. Useful features include lists of crosswalks, photos of many streets (no substitute for Google Street View, but adequate in a pinch), and references to local landmarks, to help you find your way.
• 
  Universal (iPhone + iPad)

  HERE Maps

  get itFree

  Nokia's HERE Maps brings a highly detailed global mapping system to the iPhone and iPad, and includes live traffic, public transportation info, offline maps, and turn-by-turn navigation.