Splitwise works especially well for groups that share bills regularly, e.g. roommates. Recurring bills and email reminders are especially helpful to make the monthly shakedown unnecessary. My favourite feature is the option to simplify debts, minimizing the number of payments that need to be made between people to settle up (their example: if you owe Anna $10 and Anna owes Bob $10, Splitwise tells you to pay Bob $10 directly so that there's only one transaction instead of two).

It's a great example of a "small" piece of software that does one thing and does it well.