This has been posted on FatWallet and on Flyertalk, I’ve just verified that it works. ZipCar has $25 driving credits with promotion codes linked to their specific markets. Codes are entered on the Zipcar website and are valid for only 15 days.
The great thing is that the codes are stackable — you can enter codes from each region, not just the region you are in, and you can successfully enter them all. So (6) codes yields $150 in driving credits. The codes are:
Update: It seems that Zipcar has pulled the bonus dollars out of my account, though no word from them at this point.