I use iStrelok, Winchester ballistics and bullet drop. IStrelok seems to be the best if you have or can obtain all the info, I like its versatility. Winchester is nice and simple but only works for Winchester ammo, I wish Federal would put out a similar app. Bullet drop is a much quicker easier plug and chug. I have found large discrepancies in its wind calculations compared to the other apps (and common sense). 100yd zero 200yd target with a 150gr .308 2820 MV .317bc 10mph cross wind (90*) iStrelok lists a 3.8" drop and 4.1" drift, bullet drop lists similar drop but only 1.5" drift.
Even though it is no substitute for rane time, when I can't get to the range, it's fun to use the apps to study and memorize the tables of data for my normal rounds.