I know this seems about as useful as the first time you heard about solar powered flashlights, but like the solar flashlight there is more to the story.
The problem with fire is that it puts out far more energy as heat than as light. By using thermoelectrics to generate electricity from the heat, ane use the electricity to power an efficient LED, you can produce far more light from a single candle. I think it would be fun to make a fire powered steampunk lantern, no batteries required; just toss in something burnable to power the bulbs.
The experiment serves to highlight the amount of energy we waste. The inefficiencies of our various devices are mostly expended as heat, which we use additional power to try and blow away. Research into simple, effective ways of turning heat into energy have the potential to be extremely rewarding. I intend to highlight more of them as I come across them.