Stilbenes belong to a sub-group of phytochemicals called polyphenols. They're not as abundant in foods as flavonoids, lignans or phenolic acids, which are also polyphenols. There are two stilbenes of note: resveratrol and pterostilbene. They are found in small amounts in some of the foods you eat.
Resveratrol is found in grape skins, red wine, peanuts, blueberries and cranberries. It's been studied for it's potential health benefits. Observational studies show people who drink red wine tend to have lower risks of cardiovascular disease. But it isn't known how much of that effect is due to resveratrol or if people who have lower risks of cardiovascular disease just happen to drink a little more wine than people who have higher risks.
Laboratory studies show that resveratrol acts as an antioxidant and anti-inflammatory agent in some laboratory animals, but there is very little information on resveratrol use in humans, or how much would be needed to show any health benefits.
Chances are, however, that the amount needed is too high to achieve with dietary sources alone, so any benefit would come from supplements. Please remember that dietary supplements aren't regulated for efficacy. And we don't know how much to take, or if it will do anything anyway.
Pterostilbene is found in blueberries and grapes. It's an antioxidant that has shown promise for the treatment and prevention of cancer and cardiovascular disease, but (like resveratrol) it's only been tested in lab animals. Currently, there aren't any studies in humans.
Some of these stilbenes are sold as supplements, but please speak to your health care provider before taking any types of dietary supplements.
Borriello A, Cucciolla V, Della Ragione F, Galletti P. "Dietary polyphenols: focus on resveratrol, a promising agent in the prevention of cardiovascular diseases and control of glucose homeostasis." Nutr Metab Cardiovasc Dis. 2010 Oct;20(8):618-25.
Gresele P, Cerletti C, Guglielmini G, Pignatelli P, de Gaetano G, Violi F. "Effects of resveratrol and other wine polyphenols on vascular function: an update." J Nutr Biochem. 2011 Mar;22(3):201-11.
Manach C, Scalbert A, Morand C, RÃ©mÃ©sy C, JimÃ©nez L. "Polyphenols: food sources and bioavailability." Am J Clin Nutr. 2004 May;79(5):727-47. United States Department of Agriculture. "Pterostilbene's healthy potential." Accessed June 30, 2011. http://www.ars.usda.gov/is/AR/archive/nov06/health1106.htm.