I second hotwire. I either use them or book direct through enterprise. I like priceline too but for hotels. They never can beat hotwire for cars whenever I look.
www.autoslash.com/ always gets me a better price than Hotwire or Priceline. They get you a good price to begin with but then keep searching for better deals between your booking time and your trip time and automatically rebook at no cost or inconvenience to you if the price drops (they send an email to let you know they have rebooked). My last car rental through AutoSlash was $10 a day, $20 total for two days. That includes the rental plus all taxes and other fees for a compact car.