Green lasers are perfectly legal and have been sold to the public for years. They're much more expensive than red lasers, so that's probably why you haven't seen them around much. Their selling point is they're visible at greater distances than red lasers, and you can see the beam itself in the air at night - good for pointing out constellations and whatnot (so the ad copy goes).
Of course "perfectly legal" doesn't come without restrictions. Under 5mW lasers are legal to be used outside, provided you don't point them at airplanes, cars, people, etc. Lasers over 5mW are only legal to be used indoors.