SettleUp is a tool for calculating payments to even out expenses among a group of people. It can be useful if you have roommates, each of whom pays for a certain utility. It can also help if you go on a trip with friends and everyone pays for various common expenses.
For example, if one person pays for the hotel, another for the car rental, a third for lift tickets. SettleUp gives you the list of payments that will even everything out.