Do friends constantly lend your money? Is it getting hard to keep track? Well, you should download Remember Debts! This app gives you an overview of who owes you how much money.
You get a complete list, plus a total (e.g. $5 for Pizza, $10 for Movies, Sum: $15 of Debt).

Of course you can also enter how much you owe somebody else. Another feature is, you get a copy&paste-able text, to send to your friends via text message, so it's easy to remind them of a pending I.O.U.