Do you want domestic or international car rentals?
If international, try kemwel.com or their sister-company, autoeurope.com. I seem to get the best deals with kemwel. They are based in Maine. You can cancel your reservation at any time.
I've been renting through them for years and have not had any problems. Most of their rentals seem to be through Europcar, at better rates than booking directly with Europcar. They will also beat any rate that you find elsewhere.