A few years from now, if Shai Agassi has his way, electric cars will be powered by rented batteries. Every so often, you'll pull over at a filling station to rent a new battery good for another 100 miles, like topping off the balance on your Starbucks card. Result: zero emissions. Cost: about 8 cents a mile. Agassi is just one of the big thinkers presenting at this week's Technology, Entertainment, Design conference, the annual conclave of entrepreneurs determined to make the world a better place.