Track a trip together, settle up at the end.
Travel Kitty is a shared travel-expense tracker for Android. Set a daily budget, log what a trip costs as you go, and see who owes whom at the end. Expenses can carry a photo and a place on a map, convert between currencies, and split across everyone on the trip.
It works offline: expenses are saved on the device the moment you enter them and sync when there is a connection.