Here’s a good idea — a site that

lets you enter shared bills and objectively know where you stand with your friends. When you’re on the go, you can record debts from your phone via SMS. The notion of borrowing is extended to include your personal library so you can track which things are lent out. There’s all sorts of cool features like auto-splitting bills, ties into amazon’s product lookup system for tracking your book collection, etc.

Link via BoingBoing.