Another useful website that I've discovered thanks to my housemates is BillMonk. If you ever find yourself splitting bills or covering lunches a lot and don't want to have to remember who owes money to whom or if you lend out your stuff a lot and want to keep track of who has your stuff, this site is for you.
When you first sign up, you invite or add friends to your lists using their e-mail addresses. Then whenever you say, pay a bill for the house, cover lunch since no one had cash, or buy groceries for the house, then you just add the bill (which can be a evenly split bill or an itemized bill) and BillMonk will keep track of who owes whom how much money. It displays pretty pie charts of these things, lets you see your entire loan/debt history with people, and lets you shuffle debts between people in case you owe one person money, but are owed by another.
Another feature that I haven't really used because I haven't loaned out anything since discovering BillMonk is the Library feature. Whenever you loan something out to someone, you can add that item to your library (or add anything loanable to the library from the get go) and then tell BillMonk who is borrowing it. So this way, you know at any given time who has your stuff.