Researchers at Georgia Institute of Technology have created a working prototype nanogenerator capable of generating as much as 4 watts per cubic centimeter of continuous direct current. The generators are green (to use), drawing power from natural motion in the surrounding environment. They are based on non-toxic chemicals and should be safe for use in biomechanical implants, but that's not their only potential use. From the article:

"If you had a device like this in your shoes when you walked, you would be able to generate your own small current to power small electronics," Wang noted. "Anything that makes the nanowires move within the generator can be used for generating power. Very little force is required to move them."