One item I carry that most wouldn't think of... a marlin spike. For the landlubbers, it's usually a very sharp fold out knife and a long, pointed metal instrument designed to allow one to work knots open. For this year, this is the only tool on the bike I've had to pull out (to cut off a sprung bungee that wrapped itself around my rear gearset, the spike to guide a popped off chain, and to free my chain when it got wedged between a gear and the frame).
If you live in NYC, a Metrocard, ya never know when you're 10 miles from home and can't ride back...