Mobile Passport is the best app for international travelers without Global Entry. Select U.S. airports (Atlanta, my home base is one of them) allow you to use the mobile passport as you enter through customs. As a result, you are able to breeze through customs. All you have to do is create an account ahead of time, which includes taking a passport style photo, and then completing the customs questions when you land. While Global Entry is even faster, Mobile Passport is free and you can actually do all the steps in an airplane while you are taxiing.
Operating Systems: iOS, Andriod
Developer: Airside Mobile, LLC and endorsed by Airports Council International