Candy with added or measurable amounts of caffeine
For some reason people want Energy. I'm not quite sure why they're looking for products specifically labeled that way, as my understanding is that all food contains energy. Calories are available energy from food. If it doesn't have calories, as far as I'm concerned, it's not food. But consumers, I believe, have been duped into thinking that stimulants are actually energy. They're not, they're just, well, stimulants. Stimulants make you feel like doing stuff, as the general state of humans (if you believe advertising and general laws of physics) is to be inert and bored. Stimulants, though, don't cure boredom or actually get anything done.
Caffeine is the most common stimulant and is naturally found in a product that masks the bitter and unpleasant taste of it very well: coffee. Caffeine also works as an additive in other products in smaller quantities, usually products with natural alkaloids, like chocolate or perhaps strong fruit flavors. There are some other compounds which are also grouped into energy products: Taurine and B vitamins (Niacin, B6 and B12). These have their own distinct flavors, often known collectively as the "vitamin burp" taste.