BillMonk is an online service (looks free) that lets you track who owes you how much money and who you owe money, too. What’s cool about this is that if I owe you $50, then buy you a $10 lunch, BillMonk will adjust what I owe you to $40 (unless hte $10 was a gift that I don’t report, in which case you should consider gifting me the $50, too).
We had a system like this in the house to track bills. It used MS Access and was custom built by a friend of mine. I always told him that he should develop it into a full-blown app somehow. Looks like someone beat him to it.