Check out Flyertalk.com. They have sections on each car rental agency and in there they have the list of publicly usable discount codes.

budget: http://www.flyertalk.com/forum/budget-fastbreak/176979-valid-bcd-codes.html
dollar: http://www.flyertalk.com/forum/dollar-express-renter-rewards/449064-consolidated-dollar-discount-codes-thread.html
(etc, one for each agency).

You might have to go to the newer posts and some go back a few years.