Bill Monk is a brilliant idea. It's a lightweight lending tool - perfect for busy college student. Bills come in, you list them on BillMonk, then choose people to split with.
Let's say I run an apartment, I pay the rent and utilities and my housemates pay me back. I just enter rent, then select who to split it with - and how (either divide it evenly, or enter set amounts). Then someone else buys TP and paper towels - they put in the total amount they spent and split it among the housemates. Immediately, that person owes me less - it reconciles how much I owe the other person vs how much she owes me.
It also works for lending books - I just entered all the fiction books I have (it's hard to deal with the 99 book limit (which should be lifted soon)). If you want to borrow a book from me, go down to BillMonk, then add me as a friend. Once I approve it, you can browse my library, then ask me for the book. I'll give you it and mark it as checked out.
Why didn't I think of this in college? It sure beats spreadsheets and remembering who bought what!