Wellbutrin.

I often take it for a few months during the winter to fight off seasonal depression. It also helps you quit smoking. (Or stay quit.)

As a bonus, it also has a "clarifying" effect for me. Having a touch of A.D.D., I often have trouble concentrating on one task at a time.
It takes a bit of time for the medication to completely titrate into your system but, once it does, I can have razor-sharp concentration if I try.

On the downside, however, it is a fairly strong stimulant. You have to work to get the dosage right for your individual needs. Too much and you can be jumpy and moody. Some people have sharp mood swings. This also means you have to watch out for your caffeine intake. Too much and you'll be bouncing off the ceiling!

Once I got used to it and knew what to expect from it, I found it really helps during those dreary winter months... Concentration-wise, mood-wise and it helps my creativity a little bit, too.