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
ihatemoney - Manage your shared expenses, easily
Tricount - Manage and share expenses with friends
MoneyBuster - Multi-project shared budget manager able to sync with self hosted FOSS services
WhoseBill - WhoseBill takes a new approach to sorting out money with your friends.