Originally Posted by
TheRef
Sun screen.
Lip Balm.
Some cash and/or a credit car.
Tube, co2/pump and multi-tool.
Cell Phone.
Rain Jacket if the forecast calls for rain or if the weather changes without warning where you're riding.
This is a good list. From a practical prespective, bring the same amount of stuff as you would for a ~50mi ride. There's no reason to add additional stuff for a century, and you can re-stock at rest points. I did my last century with one large bottle, because I was sure I could re-fill at all stops.