Actually, most car rental sites don't require a credit card to book. Sometimes, though, they may ask for one as a psychological trick to make you think twice about forgetting to cancel a reservation you don't plan on using, and, very rarely (and usually only with specialty vehicles in high-cost locations--like SUVs in New York City), it might be because there actually is cancellation penalty. While we currently don't support finding prepaid rates, on some of the sites we refer you to, it could also be that you're booking a prepaid rate. If you're being asked for a credit card, be sure to review the reservation rules before completing the reservation.
Feel free to contact us if you have any questions. We look forward to saving you money on your rental!