Coffee is, of course, the obvious standard answer.
I would add to that food: high-energy food. I have noticed, especially on long drives in the car, that eating energy-rich food helps as much as coffee to keep me awake.
On the flipside I have noticed when I haven't had enough sleep, and I haven't eaten a good meal in quite a few hours that I feel very tired. Once I eat a hearty meal, I feel my energy renew.