Compartimos

Compartimos is the essential application for flatmates. Note down your expenses at the moment they happen. At month's end, Compartimos will tell you how much everyone has to pay or receive.
It's designed to make data entering the easiest way possible. You can build a list of usual expenses and associate them with the person who has to pay them, and even specify it's cost. Thus, a click is enough to to note down a purchase.
Compartimos allows every flatmate to use its own phone to note down the expenses. It's very easy to sincronize data via WiFi at month's end.
Also, if you pay by credit card, take a look at Premium version, which allows to import bank activities, saving even the manual data entering. Your monthly calculations in 3 clicks!