What are some alternatives?
When comparing BillMonk and SplittyPie, you can also consider the following products
TabSplit - TabSplit allows users to keep track of their "social debts".
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!
WieBetaaltWat - With WieBetaaltWat you can manage a list with your shared expenses.
Kittysplit - The easiest way of sharing costs with a group of friends
ihatemoney - Manage your shared expenses, easily
Tabbt - Tabbt takes the hassle out of handling money among friends.