In theory if you spin an electric motor, it should "run backwards" and generate electricity. So this guy, Dan Meyer, in Minnesota built a small generator using an old gas engine and an old DC motor.

Dan says: "It is made from a 6 horsepower Tecumseh engine and a 1 horsepower 3450 rpm induction motor. I had the gas engine and frame that had been given to me from a friend. My brother had an old water pump motor he was willing to donate to the cause, and my brother in law knew where to find several motor run capacitors and picked them up for me. I got an outlet box, outlets and a pulley from my favorite hardware store and I was all set to experiment.  I worked (well...I played around with it...I can't call this work) with this alternator for the last few years, powering Christmas lights on the house, heating my Scamp travel trailer, running an electric weed wacker around the yard (the world's only gas-electric weed wacker) and other miscellaneous duties. Voltage regulation with this machine is not too bad for an unregulated mechanical device. At no load, it will produce about 130 volts. With a 1500 watt (125 volt) heater running, I see about 100 volts (or about 830 watts). To my disappointment, it would not start my furnace fan. This is because the surge current required to start my furnace fan is well over 8 amps. If you overload an induction generator, it simply stops generating.  All this experimenting has little practical value for me. To show how futile this interest in emergency generators actually is, there has been exactly one day during the last 5-6 years I had a chance to truly use this as an emergency generator when the power failed for a couple hours. I used it to power my computers so that I could surf the web. That's why this is a hobby, and nothing more."