This concept from over at Yanko Design would charge batteries using a traditional jump rope action. Also, your self-respect. Start practicing!
The charging mechanism would be placed in the handles, with the charging itself taken care of by the spinning rope. Just a few hundred hops, or 20 minutes worth, will charge the batteries 100%. Let's just hope you aren't trying to charge your pacemaker at the time. [Yanko Design via Technabob]