http://www.ebookers.com/car-hire/ - Hit or miss depending on location and vehicle type, but have saved me a decent amount compared to eveywhere else. You will have to convert the currency as they are UK based, I think they require you to prepay gas, your bank may charge a currency conversion fee and they charge a small fee for using a credit card without a PIN, so factors those in but I've personally had good luck in Florida and California after exhastingly looking everywhere else. When I was going to Denver and a few other places though they did not have the best prices. You may want to let your cc company know of the purchase in advance as I think I had a problem last time with them thinking it may be fruad. I think they are not known for the best customer service but I have never had to find out with my 3 rentals through them.
I also had good luck with Hotwire in San Diego this summer, was renting a Mini Van and prices were like $400 everywhere for the time period I was looking at, ebookers had it about $300 after factoring fees to the price but I got it at hotwire for $280 all fees included, you don't know which rental company you will get, though for some reason they showed the address to return the vehicle to so I knew it would be AVIS prior to booking and committing.