Here we go again, breaking our own rules. The Griffin AirCurve is made out of a polycarbonate, meaning a plastic. But it earns its green stripes by acting as a speaker that uses no electricity.
It's more accurate to call the AirCurve an amplifier. It takes the sound from the built-in speakers of an iPhone and amplifies it an impressive 10 decibels [source: Griffin]. There's also an incorporated slot through which you can slide a cable to keep your iPhone charged. The result may not appeal to audiophiles, but for basic users, the $20 AirCurve is a cheap, energy-free method of turning your iPhone into a mini-boom box. One reviewer pointed out that it might be more useful for amplifying podcasts or if you use your iPhone as an alarm clock [source: Kriegel].
How does it do it? The AirCurve's elegant design features a "horn" whose sinuous form may remind you of a seashell. (Think about how holding a conch shell to your ear amplifies the sound contained in your ear.) The shape of this "coiled waveguide horn" -- as Griffin calls it -- manipulates incoming sounds so that they are amplified when they come out on the other end.