iOS: When friends are trying to figure out who paid for the last meal and who owes whom the next drink, they rarely say, "I treated you to a $30 meal last time and your last treat was just $21." OU1 (pronounced "owe you one") embodies this philosophy by tracking who owes whom a drink, a lunch or a dinner.
The app works exactly as expected, going through Facebook for the login and friends-list-keeping, then allowing you to send and receive reminders as meals are accumulated. OU1 even has a currency where two drinks equals one lunch, and a lunch and a drink equals one dinner. By the transitive property, three drinks is one dinner.
These dinners and lunches and drinks can mean whatever you want them to mean. You can say that, among your friends, a coffee is a drink, or a night out boozing will equate to a lunch. It doesn't really matter, as long as you're all agreed on what things mean. The main point is that sending and receiving these drink notifications are easy, and you can quickly see who you owe from the main screen. Much easier than spending five minutes trying to remember who paid last time every time you go out to eat.
There's also an interesting factor that you would only use this app for friends that you actively track meals with. This might not be a great application for your very close friends, the ones you really don't care about who paid for the last meal or who owes whom.