Before you're old enough to either pay for restaurant bills that land on the table in front of you, or to figure out a clever way to get someone else to pay them outright, you go through a financial period in your life where many expenses are split. Restaurant tabs, phone bills, rent checks, and so on. The ongoing exercise of figuring out who owes what to whom is royal pain in the neck.
But BillMonk will run the accounting for you. You tell it when you pay for a group expense, borrow money from a pal, or perform some other social-financial action. At any time, BillMonk will tell you who you owe, and who owes you.
You can also interact with the system via SMS, which makes it easy to enter expenses and payments as they occur.
Our resident college-age intern, Andrew Gruen [posts], said BillMonk is "great;" he used a friend's homegrown tool to this previously. All that's missing, I think, is a link to PayPal, to make settling up that much easier.
Emily Chang interviewed the founders on e-Hub.