Flavonoids are what gives plants and fruits their color. They are anti-oxidants, especially in maintaining healthy circulation and can be found in just about every fruit and vegetable. In general, the deeper the hued, the more flavonoids a plant or fruit has.
Use
Depends on the plant and the flavonoid, there are many uses. I will list them as I learn about them in different herbs.
Plants
Apples, dark chocolate, grapefruit, pomegranates, and grapes to name a few.
Personal Experience
Some flavornoids can interact heavily with drugs. For example, grapefruit interacts with cholesterol lowering medicine like Zocor (simvastatin) and Lipitor (atorvastatin) or high blood pressure drugs like Procardia and Adalat CC.
Flavornoids are polyphenolic.