Some overhyped "natural" bullshit product found in certain fruits, most notably grapes and some obscure japanese algae, which might explain the French paradox, but not really, seeing as how many other countries happen to consume about as much wine.
Supplements of this shit are sold, but no one seems to give enough of a fuck to actually do any form of remotely meaningful research. Yahoo doesn't count. Besides, it hasn't been around for long enough for side effects to be seen, and so, googling resveratrol up will lead to ~3,000,000 results, all claiming that the thing can cure about every possible ailment known to man (and even some unknown). Of course, it'll eventually fail, as with most products of its kind, when its male users start growing hair on their dickheads because of it.
Mike: My uncle bought some resveratrol.
Mike: What a dumbass! He got caught into some ponzy scheme pyramid bullshit... again.
Mom: Mike, who are you talking to?
Mike: Nevermind you hag.