In addition to what mia provided, I would recommend looking (a) in the individual rental car forums to see if they have a master thread listing all the promotions (as not all the very best promotions are easy to find on the airlines' own websites), and (b) checking this site:

http://frequentflyerbonuses.com/

There, click on Airline Offers if you want to see bonus offers sorted by airline, or Car Rental Offers if you want to see bonus offers sorted by car rental agency.

And, btw, I suggest you check all of these, as each one may be incomplete. Now that there is no longer a master thread for car rentals (since Evan! disappeared) in this forum (or elsewhere on FT), you have to look several places to have a better chance of finding the very best offer for your needs.