The required stuff by the company:
Two 2" jepps (over stuffed and ready to explode)
One 1" jepps "trip book"
One 757/767 aircraft operations manual
One flashlight
One headset (telex 750)
Dozens of Jepps high and low charts

What I add:
Pilot contract (slim version)
clipboard (for ETOPS flight plan)
Plotter for ETOPS chart
Highlighters (for ETOPS plotting chart)
Canon digital camera (small one)
Sunglasses
Altoids and/or gum
Energy bars
Pictures of my family
Star charts (for something to do in the middle of the night over the North Atlantic)
Timer (to calculate 757 center tank fuel at 1000lbs, so I can turn the center fuel tank pumps off, to keep the airplane from exploding)
A book and/or magazine