When I was on my trip to Yellowstone, several of my classmates got into a discussion about useful photography apps for a smartphone. I offered to share mine and how I use them.
First, there are probably free or at least cheaper versions of these types of programs. I started down that path and then it dawned on me that I spend thousands on equipment and trips, and if an extra $20 can make the difference between getting a good photo or not, then it was stupid not to get a high quality app.
TPE (The Photographer's Ephemeris) $9, lets you know everything about the sun and moon rising and setting. Which direction, times, angles, twilights (there are 3), shadow lengths, and all from any location and date. It is a great tool for searching out where to be for the perfect sunrise or sunset, while the sun is still overhead.
TrueDoF Pro $6, Did you know that when you are shooting a 400mm lens at f/4 at a subject 100 feet away, you only have about 4 feet of depth in focus? That means if the back of the bison is in focus, the front won't be. This app makes it easy to figure out the DoF and the
hyperfocal distance for any combination of lens, f-stop and camera. Crucial for landscape photographers but useful for everyone else.
Long Exposure Calculators $4 This app has a ton of useful tips and semi-automatic calculators for shooting anything with longer exposures, including flowing water, moonlight exposures, star trails, fireworks, .... It also helps figure out what ND filters you should use and the effect they will have.
Star Walk $3 Not purely photography, but great for identifying all the stars and planets. Like TPE above, it has tons of good info about sun and moon rises and sets.
iBird $20, I am a bird photographer and it is great to have a high quality bird book in my pocket at all times when I am out shooting.
If you have others you would like to add, please put them in the comments. All the hyperlinks I added are for Apple phones and tablets, but I believe each of these apps is available on the Google Play store as well.