I have some coupon codes, each good for a free day's car rental from National Car Rental.
These are good in the U.S. only, and only on regular cars (no SUVs, minivans, etc.).
Taxes and service fees are additional, and are often pretty high, but you will pay no base rental charge.
Up to three coupon codes can be used per rental.
They expire on June 25th, 2011.
When you book on National's Web site with these, you will get the message "You have entered a coupon that may be required to be presented at the time of rental."
These are purely electronic codes and there is no physical coupon. Most agents at the rental counter know this and will not ask you for a paper coupon.
To avoid having any problem with a clueless agent, you can join National's "Emerald Club" which lets you skip the counter and choose your rental car.
There is a signup link on http://www.nationalcar.com/
Looking to get some BTCs in exchange for these, please PM with offers!