There are several airlines and a hotel that is offering bonuses through car rentals.
One of the better ones is with Budget Rental Car where you can get up to 10,000 American Airline points for a 10 day rental. The promotion ends soon on Sept 30, 2012 so if you live near a participating location that offers cheap rentals for 10 days, this is worth the cost since buying 10,000 AA points would cost $325. Make sure you use coupon code MUAZ013 and BCD #U072421 to save 30%.
Another one I received through email is with Southwest Airlines where you can get up to 2,400 Rapid Reward points for a 4+day rental. Remember to use K024769 in the AWD field, MUAA037 in the Coupon Code field to save 35%, and provide your Rapid Rewards account number to receive your points. Promotion ends November 30, 2012.
A smaller promotion is with Priority Club where you can earn 1,000 bonus points and save 35% on a one or more day rental on any car. Promotion ends Dec 31, 2012. Valid only in the US, Canada, Puerto Rico, and Europe.
New Girl in the Air has a post where you can earn 4x American Airline Miles with Hertz. Promotion ends Nov 30, 2012.
So if you have an upcoming rental, make sure you take advantage of these great car rental promotions.