A Norwegian solar power company named EnSol, with the help of the University of Leicester, is in the process of developing a transparent material that could be used to harvest solar energy and convert it to electricity. The material could be applied as a very thin film, possibly even sprayed on. While a certain amount of sunlight must be absorbed by the film in order to produce electricity, about 90% would still pass through. This means it's perfect for windows on homes, offices, and cars.
The benefits of a solar tint film would be two-fold. First, obviously, you get free energy as long as the sun is shining. Use it to run accessories or charge batteries, cutting back on consumption of other fuel sources. Second, depending on the amount of sunlight absorbed by the film, the equipped car's interior (or building interior) can remain cooler, also reducing the amount of energy required to maintain environmental comfort.
Buildings are a great application for the transparent film, due to their abundance of surface area. The effect will be diminished on a car, but as the efficiency of the material improves, it could make a worthwhile impact on the energy consumption of a vehicle.