I've used allspaincarhire.com a couple of times which I think is related to carjet, above. The rates beat all the big players by a country-mile. I think what they do is "buy" excess rental inventory from the big names for a song and package it out, thus they'll tell you which of the companies' desks you have to go to downstairs at AGP to pick up your keys.

Can't remember exactly how much I paid last time I was there, but I do recall thinking that I got an S class Merc for a week for a lot less than Avis/Hertz et al wanted for an Escort class.