I've worked a lot on a Bukkit plugin that adds a few more things to the sky, and so year I have a working helicopter and parachute. It is not entirely done, but I will release a part version of it soon. The helicopter is basically just a minecart, but in whatever direction you look, it goes. If it is touching the ground, then you have to go up a bit to move around freely. This is to prevent people using the helicopter as an easy car plugin. I want to have it run on fuel, but I'm not sure exactly how I will do that. Right now, left clicking will shoot fireballs in front of it, and I'm thinking that maybe right clicking will shoot mini fireballs, the kind you get from a dispenser. The helicopter explodes if shot, and to craft it is a little expensive, to keep everyone from flying around randomly. It takes a block of iron, one stick, and three diamonds. I'm thinking of adding a sort of helicopter animation, with a squid spinning above the player. Another really fun thing is that if a creature rides it, the minecart will actually move in whatever direction they are looking. High speed zombie pursuits! Unfortunately, they don't always look up. The parachute is just a renamed block of wool, and is crafted by using three wool, two string, and a leather chest plate. If you are in the air and right click with it in your hand, it will slow your descent. I actually have a sort of parachute that goes above you, which is basically two wool blocks.
I also recently made a car that can go up and down blocks, and am working on a tank, too. I'm thinking of having this for a custom server, where there are two worlds. The main world will be like a bunch of houses, and the second world will be where you can mine and get resources, and while doing so certain blocks will give you money. You can use the money to buy houses!
I hope to add more functions and other vehicles to the air, but you can download what I have here.