http://www.carjet.co.uk are always the cheapest to hire your car from. They use other firms and just act as agents, but are always cheaper than going to the firms direct.
Rental firms around Mojácar tend to be cheaper than most at the airport, and they will usually arrange for you to pick the car up at the airport. For Almeria Airport, we have used Mo-Car rentals of Mojácar, you can contact them on 00 34 950 478 307 , Fax 0034 950 475030, or email Ambrosio Flores at mo-car@larural.es , they also have a website at http://www.mpm-mocar.com/eng/index.htm . When we checked prices, they were much cheaper than anyone else in Mojácar (EUR 285 for 11 days), and they met us after baggage claim in Almeria Airport, which saved all the waiting around queuing with the big hire companies (we spent over an hour doing this at Alicante Airport once). They have an office at the bottom of the road that the apartment is on, next to Polansky Associates.
Another cheap one for those using Alicante or Murcia airports is http://murcia.spain-car-rentals.com/
Otherwise, you can try the following links for car hire.