I am looking for a good way to use my iPhone, Mac, and/or .mac account to keep track of small amounts of money that my friends and I owe each other. Any suggestions? Here's a typical scenario: Friend: Hey, let's go to this concert! Me: Sure, I'll buy the tickets and you can pay me at the show! (two weeks later, while shopping) Me: Crap, I don't have enough cash for this must-have thing Friend: Oh, I'll buy this for you now; then I'll owe you less at the show (two weeks later, at the show, trying to figure who still owes whom what) Me: ? Friend: ? This happens a lot. We're not trying to scam each other of course...we're just having a hard time keeping track of and remembering this stuff when we're out.