The MetalCell (which originally appeared in an issue of Popular Science) is all over the blogs, but I cannot find where the product can actually be purchased (or any details about the voltage produced by the cell). Anonymity withstanding, the idea of an emergency power source that can run off of saltwater or urine is pretty cool.
The technology behind the MetalCell is said to have originated in South Korea for use by the military. The battery is powered by the electrochemical reaction between the magnesium plates in the cell and the sodium contained in salt water or urine. In theory, the MetalCell can power a laptop for 4 hours, though the actual power output is going to depend on the sodium content of the fluid in the cell. The batteries are best suited for use as an emergency power source since the magnesium plates will eventually corrode from use. In the meantime, the cells are small and easily stored away until they are needed.