Maca is a root vegetable with medicinal qualities. It helps balance out the effects of stress by aiding in the regeneration of the adrenal glands. It helps you sleep better by lowering your cortisol levels, which of course will increase your energy level when you're awake!
Maca also helps your tired muscles to regenerate faster, which will lead to more strength gains. I throw this powder into my morning smoothie and into my protein shake after a workout.