ELFSH alows you to share costs. It keeps track of a total for every person in a group. If you want to buy something "on the group" you can enter it in ELFSH and the price will be split over every participant. After some time you can chose to pay the differences.
ELFSH alows coordinating eating togher. If you live with a lot of people there is always competition for the kitchen, and if you do cook for multiple people coordinating every day is a pain. ELFSH alows easy coordination even for large (10+ people) groups. Simply set yourself as "E" if you want to eat and "K" if you can cook. No need to have an anoying chat conversation each and every day.
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!
LetsPayTheBill.com - Free online tool to calculate the distribution of expenses during a group activity.
WieMee - With WieMee you can easily share expenses with friends or colleagues.
Kittysplit - The easiest way of sharing costs with a group of friends
Tricount - Manage and share expenses with friends
WieBetaaltWat - With WieBetaaltWat you can manage a list with your shared expenses.