Build a stationary bike energy generator
This build takes skill, but the reward is worth some (literal) sweat equity: Your technology will now run on calories burned while you pedal your new stationary bike.
Bike Frame (Free or cheap on Craigslist)
Assorted Wood and Wire
12V Lead Acid Battery - $25
DC-AC Inverter- $32
DC-DC Battery Charger- $84
24V DC Scooter Motor- $40
Fan Belt $16
Fan Belt Pulley $7
This is a project that calls for in-depth planning, so start here for step-by-step instructions with images: http://www.instructables.com/id/DIY-Bike-Generator
The back wheel of the bike spins a DC motor via fan belt, the motor is connected to a charge controller, the charge controller charges a lead-acid battery, and the battery is then connected to an inverter. You can then plug your phone into the outlets of the inverter! The majority of this project involves the assembly of the bike platform and the attaching of the motor and fan belt.