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
Splittr - Splittr helps you to split group expenses and is so easy to use.
Tricount - Manage and share expenses with friends
WieBetaaltWat - With WieBetaaltWat you can manage a list with your shared expenses.
LetsPayTheBill.com - Free online tool to calculate the distribution of expenses during a group activity.