Evolution is the word. You have the big boys of the rental car business and there is EV Rental.
The company was founded in 1998 and its fleet is made of hybrid vehicles only (Toyota Prius, Honda Civic Hybrid, Toyota Highlander Hybrid).
EV Rental currently serves only the West Coast ( airport locations in Phoenix, Las Vegas, San Diego, Oakland, Los Angeles, San Francisco) and its rates are competitive.
As an added benefit, driving a hybrid vehicle allows you to use the car pool lane in California even if the driver is the only occupant.
Thanks to Springwise for bringing them to my attention