bobricius designed this low voltage step up converter module with white LED:

40mV module for body heat powered white LED

Features

• startup voltage 40mV
• requires only peltier element (not included)
• white LED

HOW IT WORK

• Peltier module generates very low electricity from temperature difference.
• Has two sides. If you keep COOL side (with aluminium cooler) at about 20 degrees and HOT side you heat with your body (fingers, forehead) with temperature etc 37 degrees you get about 40mV.
• This converter module increases this very low voltage to power white led.

Here is a video of the project: