TabSplit allows users to keep track of their "social debts". So if you go out and share a tab it's easy to split it up between your friends and know who has paid the tab and who owes you the remaining money.
Tabs (Split up itemized bills between friends - e.g. restaurant bills - in this android app you can take a picture of a bill, split up the items and later digitalize the items on the website tabsplit.net).
Payments (Record loans to your friends).
Joint Payments (Group investments where the cost is shared equally across all participants, e.g. if you share the hotel bill).