Mike Davis built this DC generator from an old lawn edger and a permanent magnet DC motor. He writes:

The inspiration behind this generator was to have a way to charge up my battery banks on days when the weather is cloudy and calm, when my solar panels and wind turbine aren’t providing much energy. It was a really quick, easy and cheap project. I got the lawn edger for only $5 at a yard sale. The permanent magnet motor came from an auction for another $5. The total cost of the project was only a little over $20! So far I have the output of the generator up to 10 Amps at 12 Volts. Further tinkering may improve that. My eventual goal is to convert the generator to running on wood gas from my biomass gasifier project.

Mike’s website has a bunch of really cool projects on it, like the above-mentioned gasifier.

My Improvised DC Generator Build