Avis offers 500 miles per rental for British Airways (and no flight in conjunction is required), also have a promotion these days which gives 1500 BA miles for the first rental.

Alamo offers 250 miles for USAirways (no flight is needed).

For longer rentals there are some triple mile offers (which translates to 1050 miles for a weekly rental) - National and Budget are offering triple AA miles, Alamo is offering triple miles for United PE 1K members. You may have to ask for a higher rate to get the triple miles. In my case it was not worth paying the extra money to get the extra 700 miles.

The Southwest offer mentioned by Tino sounds good, but keep in mind the expiration policies of Southwest.