Engine: FaceBook

OmniSpense.com features: It displays who owes you and vice versa (and how much $), and it displays all recently added/changed expenses that involve you. From the OmniSpense.com home page:

OmniSpense.com can help you organize both your personal and shared expenses and bills. This FREE service was originally designed for college students living with roommates, but it also works well for anyone living with roommates, for splitting up large group lunches among business colleagues, and even for tracking small business expenses. In fact, we the "OmniSpense.com Guys" use our own product to keep track of both our living expenses and our business expenses. With OmniSpense.com,
* easily record shared expenses with anybody
* instantly see who owes you and who you owe
* keep track of your own personal expenses
* all for free!