Well sort of free, you still have to buy the thing.

I've just come across the 'npower peg' ... it stores the energy that you produce while riding, walking, etc so you can use it to charge or run your phone, gps, adult toys or whatever you like. I'm thinking it's got to be a better idea than producing energy via a stove.

http://www.npowerpeg.com/new-how-it-works

£200 and likely the size and weight of a house brick

EDIT: $200 so c.£130 and 400g. Still not sold

Bit heavy and at first looked interesting but there are very mixed reviews online to the point where some say it is a scam.