A few of my friends have launched the beta of a Web site from which you can send informal notes to track debts (in cash, goods or deeds). Say a business colleague buys you lunch and you want to combine a thank-you note with an offer to reciprocate. IOU Note makes it easy to do so. Interesting idea, and I think they've executed it well, although I'm of course biased. I did send them a bunch of suggestions and I know they're furiously working to improve the service. It will be fun to watch their progress.