First Aid kit
I know we don't like to think about road rash and other things that can happen. But it makes a lot of sense to be prepare. Most First Aid Kits are just too big to carry on a bike and not really made for what happens when you fall or if you are like me when you make roadside repairs.
The Adventure Medical Cycling/ Road Rash kit comes in a small waterproof bag that will slip into a bike bag and not steal all of the space. It certainly isn't complete but has what you need most often and is something you should keep with that multi-tool, patch kit and tire bars you carry.
They are on sale now at Cambria Bike for $5.95, just go to Road Rask Kit.