The Mousesavers.com site has some useful discount and coupon codes for saving money on rental cars.
Be sure to call the agency to ask about extra-driver fees. Some of the “contract IDs” on the Mousesavers site provide for a free extra driver, whereas others allow for up to 8 free drivers. (Alamo, for example, charges $7/day for each additional driver.)
Also, be forewarned that spending two hours trying every possible permutation of coupon code, contract ID, and rate code is a terrible way to save $10.