Going on vacation doesn't have to mean leaving your green conscience at home. The car-sharing service Zipcar rents hybrids cars in five U.S. cities, Toronto and London. A few specialty companies offer rental cars that run on biodiesel fuel, a clean-burning substance derived from renewable sources like vegetable oil. Bio-Beetle rents eco-friendly cars, ranging from Passats to Jeeps, in Hawaii and Los Angeles. A week's rental in L.A. runs from $200 to $300. And competitor EV Rental Cars has started to expand beyond the West Coast.