There don't seem to be any raelly fabulous ones. Avis offers a coupon for one free rental day after you've rented for 15 days, the coupon is good for about 4 months, so that limits the deal somewhat. Plus, they'll throw in a free upgrade occassionally. They also give you iTunes credits--I think it works out to slightly less than one song per day of rental, which is kinda nice. Many of the rental car companies will credit your favorite airline program for frequent flyer miles (one mile per dollar spent), but this is really peanuts, and many charge you a buck or two to cover their admin costs for doing it.
Of course, rent using a credit card that offers some type of reward, and that offers good insurance coverage for the rental car. That saves a lot on the ripoff policies they sell at the counter
If you are paying for the car with your own money, skip the loyalty reward programs and just go for the best price. One benefit of going with the lower-tier folks (e.g. Enterprise, Budget, etc)--they tend to have fewer cars on the lot in the smaller locales. If they don't have the type of car you reserved, you'll get a nicer one. That has happened to me often with Enterprise, but far less often with Avis.