It's called Modafinil and it's in a class of drugs called eugeroics. It's a stimulant in the same vein as caffeine and adenosine, but the difference in how it works the brain means it doesn't have the same side-effects that the others do.
The way caffeine and Adenosine work is that they block certain receptors in the brain that react to dopamine, which makes it slow down. This means, for example, that coffee does not give you energy, but rather, doesn’t let the brain realize it needs more energy. This is why people crash after caffeine. The effect goes away and all the tiredness catches up to you. Modafinil doesn't have that problem.
No one really knows how it works yet, but it seems that instead of blocking the brain's dopamine processing, it just slows production of dopamine. On top of that, it also prevents the re-uptake of another neurotransmitter called noradrenaline, which triggers sleep.
The end result, and one of the most mysterious, is that modafinil doesn't trigger sleep debt. People who stay awake for a day or two on modafinil don't report a need to catch up on sleep after the effect wears off. They can sleep the normal amount!
There's a lot more to say about this, so check out the source if you're interested.