You know those times you are staying in a mountain cabin with friends, and you go outside at night? It is so much darker than city nights and you gaze together at the billions of stars. Then you try to show off to each other, by naming constellations. Everybody knows Orion, but otherwise you are mostly just guessing.
That's why I love Star Chart. Using the GPS in your phone and other sensors, it can tell where you are and where the phone is facing. So point the phone towards the star, planet, or constellation you want to identify, and it tells you all. Mountain cabins often lack mobile phone reception, so gratefully this app works offline. Not only is it useful, it is beautiful.