iOS/Android/Web: Between eating out, grabbing coffees and sharing bills, you can rack up a lot of IOUs in day-to-day life. Splitwise helps you manage them with a simple ledger, and lets you pay back your debts via PayPal.
We've mentioned Splitwise a few times before, but it has matured a lot in the past year. It's now a fully-fledged IOU manager, helping you and your friends keep all your debts straight and easily payable.
You can add individual friends from your address book or set up a group, like your housemates (They don't need to be Splitwise members for you to manage your IOUs, but it's a lot more useful if they are). It even includes a handy bill splitter, perfect for when you're out at a restaurant and don't want to do the maths yourself (though only the web version lets you split the bill item-by-item). When you want to settle your payments, you can just send a quick PayPal payment to your friends right from Splitwise.
It may seem like overkill for the occasional lunch or after-work drink, but if you share expenses with your friends often, it can be a good way to keep track of who owes money to whom.
Splitwise is a free download for iOS and Android, as well as a free webapp for any platform.