Hertz has been running an offer most of the year that earns 4 miles per dollar for all rentals in the US, Canada, Mexico and Europe for all rentals up to 4 days long and 6 miles per dollar for all rentals longer than 4 days. It was due to expire in August but it appears Hertz is extending it through the end of the year.
I happen to be a Hertz fan so this one sets up nicely for me. AVIS also has a current offer of 2,000 United MileagePlus miles per rental that I think represents a fair deal in certain scenarios.
National is also in the process of bringing back One, Two, Free which is good for people seeking free rentals on a tighter budget.