Lutein

Lutein is one of a class of nutrients called carotenoids. It is not converted into anything else as is beta-carotene, but lutein has similar effects; it is a strong antioxidant.
All of the antioxidants have a particular part of the body that they affect most; in the case of lutein, it is the eyes.
Lutein is found in several foods; the principal sources are green leafy vegetables and egg yolks, although there is a small amount of lutein in all animal fats. It is used commercially as a colouring agent (a harmless one, for a change!) in eggs - it is added to commercial chicken feed and the lutein is incorporated into the yolk, making it darker yellow. Lutein is not used for other colouring purposes, because it is not particularly stable - especially in acid conditions or in the presence of other colouring agents.
Lutein is found in large amounts in the macula, the central high-definition part of the retina that contains most of the cones (the colour-sensing cells) in the eye. This is thought to be because the cones are especially sensitive to oxidation and damage caused by light, particularly light at the blue-violet end of the spectrum. Lutein is reddish-orange, or yellow in low concentration, precisely because it absorbs blue light.
Lutein is therefore very useful for those, such as the elderly, who might be susceptible to macular degeneration - at least the dry form. Wet macular degeneration is caused by abnormalities in the blood vessels of the retina, hence is not helped much by lutein. However, the dry form is more prevalent anyway.
Lutein has no known side effects, except that caused by extremely large amounts of any and all of the carotenoids; the excess is deposited just under the skin, leading to orange/brown colouration - which may help deepen an existing tan in any case, and thus might be thought actually desirable. In any case, one would have to take a lot of lutein to cause this effect - amounts much larger than found in most supplements.