Splitwise is a web based service for managing IOUs between small groups of people who regularly meetup. The app tracks who spends what for whom so that everyone is on the same page and can see what is owed. It is also very clever in sorting out IOUs in that any if someone else buys something for someone they owe money to, their debt is cleared for that amount.
The app also helps to manage the regular payment of bills, ensuring that bills of all kind are paid off, though this is less useful then the group function above given the prevalence of other methods. And while offering a web based service, it also offers applications for iPhone and Android so you can access and use the service anywhere!