A colored, flavored solution of organic and inorganic acids. The fact that diet Pepsi in general, and caffeine free diet Pepsi in particular, serves no nutritional purpose can be observed from the fact that ants will not touch it. Leave a can of Pepsi, or Coke, if that's your thing, outside and within minutes it will be covered by a frenzied, teeming riot of little black dots eager for their piece of the high fructose corn syrup pie. But leave out a can of diet Pepsi, with or without caffeine, and it will sit sad and alone, a neglected wallflower next to her pretty and outgoing sister.
Ants aren't dumb, though individually mindless. They know that diet drinks are pointless and empty even if they do taste sweet. The caffeine free variant does have one use, though, and that is to break a soda habit. If you, like so many, cannot live without your fix caffeine free soda is useful because it helps to break the mental association between the soda (mmm... sweet) and the drug (sweet!!) It becomes frustrating once the withdrawal symptoms kick in, as repeated doses of caffeine free soda refuse to satisfy the craving.
Or end the headache. Caffeine, besides being a diuretic is also a vasoconstrictor, which means that quitting soda will expand your blood vessels for a time. In particular it will expand the blood vessels around your brain, thus giving you a glorious headache that will not go away for a couple of days.
There is, as far as I know, no Caffeine Free (Undiet) Pepsi. No doubt someone out there would buy it, but for now if you want sugar jitters but not caffeine jitters you'll just have to add the white stuff yourself.
ps. As jack pointed out, there does indeed exist a Caffeine Free Pepsi, with all the high calorie goodness you'd expect from that brand of black sugar water. God Bless America.