Carnitine and Vitamin E
OK, so we're cheating a little bit on this one -- carnitine isn't a vitamin. But while vitamin E has shown in some studies to improve sperm activity, there's much better evidence for carnitine, something your body uses to transform fat into energy [source: NYU Langone Medical Center].
It's not considered an essential nutrient, and you get what you need from your own body -- your liver and kidneys make it for you (although you can use it as an excuse to eat more steak, ice cream and cheddar cheese if you want -- they're all good sources).
Even though you don't need any more than your body produces, supplements of carnitine have shown to help with sexual dysfunction, the effectiveness of Viagra, and sperm quality and motility. It's not definitive evidence -- a couple of studies contradict each other. But science has its eye on carnitine, so stay tuned.
As for vitamin E, it has enough other benefits for you to love -- it's an antioxidant, it strengthens your immune system, and keeps your blood from clotting dangerously, among other functions your cells need. And you can find it in foods that are good for snacking, like almonds, peanuts and sunflower seeds.