Hybrid cars are all the rage, from the Toyota Prius to the Chevy Volt and the Fisker Karma. The same kind of technology that makes these cars quieter, cheaper and more efficient is now finding its way into aircraft.
EADS and Siemens teamed up to develop the DA36 E-Star aircraft, with the goal of reducing noise and emissions by 25%. The plane's engine is conceptually similar to the engine in the Chevy Volt: there's a conventional gas-powered engine along with a battery, but only the battery actually drives the motor: the gas engine is there exclusively for battery charging. Because of weight constraints, the E-Star can't get very far on battery power alone, but the idea is that the battery will be used primarily for take-off, when power demands (and noise) are at their highest.
Aircraft, of course, aren't going to benefit as much from a hybrid system as cars do, since in general once you get off the ground you simply point yourself in one direction and let the engine run at a constant RPM until you're about to land. There aren't the same kinds of stop-and-go opportunities to save fuel, much less regenerate energy.
This isn't to say that a hybrid aircraft isn't a good idea, though, and it's true for many of the same reasons that hybrid cars are a good idea: you get a bunch of the efficiency benefits of an electric system without having to worry about range anxiety, since you can always just kick the gas engine in if you get low on power.