Google Sky Map
Google Sky Map [Free] is an app made to complement Google Earth. Essentially it is the same thing, but with stars, constellations, and planets.
Star Walk 2
Star Walk 2 ($2.99) is an interactive augmented reality stargazing app that lets users explore what they are seeing in a 3D simulation.
$0.99 for standard version, $7.99 for plus, $19.99 for pro.