National Car Rental is running rent twice get a free rental day between August 22 and January 31. Registration required and there’s no retroactive credit for rentals taken prior to registering (but then if you’re reading this contemporaneously as I write, the promo hasn’t started yet, so you’re not worried about this). Unfortunately the promo applies only two rentals of mid-size cars or larger, for two days or longer.

Free rental days will be awarded beginning September 2, and future emails with free rentals will come within a week or two after each free rental day is earned. Free Rental Days are non–transferable and are valid for use through June 25, 2012. No more than three free days can be used on a single rental.

The promo apparently doesn’t apply to existing reservations, if you have any reservations already booked for August 22 or beyond you may want to look at cancelling and rebooking.

The promo also includes a referral component, if you refer someone to the offer and they sign up and rent, you’ll earn rental credits (capped at 10 rental days).

I’m a big fan of Natioanl, at least at the base level. See, Hertz and Avis probably have better (invitation-only) top tier levels. But for a general member of the program I think National’s Emerald Aisle (“pick your own car”) is probably the best thing going. Avis usually has more generous promotions, but this strikes me as the most lucrative promotion for two-day rentals. A three day rental for 3000 US Airways miles with Avis could be more or less generous, depending on how you value US Airways miles versus a free rental day.