Originally Posted by
robyr
pump/CO2, patch/tire kit with an extra tube, and a helmet.
EDIT: And if you are going to ride at night at all, you need at the very least a rear and front light set.
I'd consider these essentials. Rest are a matter of personal preference. I commute 56+ miles per day, run no fenders and wear a messenger bag for example.
Add a solid small tool kit on top of the patch/tire kit and some basic mechanical knowledge.
Oh, and don't cheap out on the lights if you're riding a lot at night. You want to be seen