Correction Appended: November 6, 2007
How's this for eco-friendly: a battery that runs on sugar. Sony's bio cell, unveiled in mid-2007, uses glucose-digesting enzymes to extract electrons from any sugary solution (as with other batteries, the electrons flow around the circuit to generate electricity). Connect four of the 50-milliwatt cells, and you've got enough juice to keep your MP3 player humming.
Available Prototype
sony.net
The original version of this article stated that the Sony bio cell prototype was unveiled in mid-2008. It was actually unveiled in mid-2007.