The Refill My Metrocard app allows you to find out how much more you can add to your Metrocard for the Metro in NYC (that is, the train that goes underground). Just type in how much your current balance is and you'll get options of how much to add so you don't have a leftover balance! 

Works quickly and requires no internet connection so it'll work underground; just fire up the app when the MTA machine tells you your balance.

[What is a Metrocard]

A Metrocard is a plastic card used to pay for a Metro ride. They are compatible with most machines that use Metro fares.