http://en.wikipedia.org/wiki/Drugs
First picture caption: "Coffee is the most widely used psychotropic beverage in the world. In 1999 the average consumption of coffee was 3.5 cups per day per U.S. citizen."
Caffeine is a CNS stimulant\adenosine receptor antagonist. It increases wakefulness\helps focused thought but impairs memory. It's a stimulant drug, not a very good stimulant drug, but the most commonly used one with a large social presence.
Vastly superior safe legal stimulants imo: Rhodilia Rosea, L-Tyrosine, Cocoa, Galantamine, Spirulina
Others legal ones I know a lot of people swear by: Piracetam. (and relatives such as Oxiracetam, Aniracetam) Becoming decently common among college students.
Adderall is also EVERYWHERE you look in competitive colleges, but long term amphetamine use is a very bad idea in terms of burnout unless you are super human like Paul Erdos, and its a controlled substance.