1) Helmet - This really is a necessary item. Everyone crashes from time to time and the helmet can prevent serious injury to the control center of the body.
2) Mirror - All summer I rode my new road bike without one of these and a couple of times drivers, who feel they do not need to give a bike room, whizzed by me and almost sent me into the ditch. The rear view mirror has decreased the amount of times that has occurred.
3) Spare tire and repair kit - You never know when you will have a flat, they always seem to occur at the most inopportune time.
4) Pump - If you get a flat you better have a way of pumping it back up.
5) Multi-Tool - Other things do go wrong with bikes while on the road and a tool in the tool bag can make the difference between thumbing a ride or finishing the ride.
7) Headlight and Taillight - If you are riding before or after the normal hours of daylight you certainly want to be seen by the bigger less maneuverable motorized vehicles.
8) Bike lock (U-bolt type) - Because people really can not be trusted, leaving ones bike unlocked is a sure fire method for getting that newer nicer bike.
9) Gloves - A good set of padded gloves really cut down on the numbness in the hands and make long rides much more comfortable.
10) A little food