WhoseBill is a a free online application that makes it easy to track and settle IOUs and debts between friends. It works a bit like a social network, but with the purpose of taking the hassle out of managing money and remembering who owes what to whom.
WhoseBill is simple to use, but uses an advanced algorithm to offset debts and payments against each other, enabling you to stop worrying about each and every IOU and simply see how much you owe or are owed overall for each of your friends.
WhoseBill also brings the concept of "pools" which let you easily offset debts and payments between multiple people For example, you might have a pool for your flat which you rent with others, allowing anyone to pay the bills, and simply manage who is ahead or behind, rather than trying to pay back individual shares of each.
When you are ready to settle up with a friend, or in a pool, WhoseBill does all the hard maths for you, working out the most efficient repayments to cancel out all the debts, and then helps you manage the process of getting everyone to pay.