I've rented cars in the U.S and in other countires. In Austrailia last spring, they offered a deal where you could bring the car back needing Gas so long as you had taken advantage of their offer to let you pay for a tank of gas up front at less than the going rate per litre. The idea I'm guessing is that most people end up bringing the car back with at least some gas in it so the company would only have to fill it part way. It could be a good deal if you were able to time it right so that the car was running on fumes rather than return it with a partial tank.
The thing I learned about renting cars is that you need to be absolutely sure (in writing) about your own insurance coverage when you opt not use the insurance the rental car offers. Be absolutely sure your insurance covers you in the State or Country you rent the car in. Same goes for the Insurance offered by your credit card. We had a fender bender with an old man who ran into us while in Austrailia and we had to pay the damages to our rental car upfront before leaving the country until the matter was settled. We were assured over the phone by our agent before we left the U.S that we were covered but found out when we got home that if it had been our fault, our insurance wound't have covered us. The fine print on our specific policy, revealed that Austrailia wasn't one of the coutries included in our coverage. That could have been a mess especially if there'd have been injuries.