Hi, I co-founded BillMonk. Thanks for writing about us! I swear, we never evesdropped on you! Really. :) Both the lending library and the social money aspects of BillMonk are ideas that a couple of people have told us they also had. I guess the problem of tracking borrowing is such a clear case where computers can solve an everyday issue. I'd like to think that BillMonk's main value is that it provides an easy and friendly interface for doing so. As for the friendship-killer issue... we want to help friends, not to hurt them! If using BillMonk is adding an unnatural level of strain due to everyone penny-pinching, don't use it! Most people use the shared-bill functionality to evenly divide a bill - we do the math. (You can be exact if needed, of course). We have received a ton of love-letters from users, especially from roommates, saying that the ability to be precise has defused the everyday stresses of worring about fairness.