I'd say bring a patch kit (those nice self sticking kind), tire levers, a spare tube and a pump. Those small mini pumps are cheap (and don't work too well, but if you have a flat getting it fixed is better than walking).

A mirror (either one mounted on the handlebars or on your helmet). The ability to check on traffic coming up from behind is great without having to turn your head and take your eyes off the road. I road a borrowed bike for a week and forgot my mirror and that was the thing I missed the most.

I also have a small pack that I can access easy that has my ID, a cell phone and a pen. That way if I get hit, someone knows who I am, or if I need to call for help or the police it's easy to get to and a pen to write down license plates if someone does something that deserves a call to the police.

One other thing I picked up this year was an air-siren (small one in the marine/boating section of the sports/auto store). These are about $8 and if a car cuts you off of you need to get a drivers attention these work great.