Splitwise - Splitwise is a free tool for friends and roommates to track bills and other shared expenses, so that everyone gets paid back. On the web, iPhone, and Android!
Kittysplit - The easiest way of sharing costs with a group of friends
Tricount - Manage and share expenses with friends
Splittable - Shared living made simple
ihatemoney - Manage your shared expenses, easily
WhoseBill - WhoseBill takes a new approach to sorting out money with your friends.