I've been digging and digging, and I can't seem to find any free food for Cinco de Mayo. The closest thing I could find is a free Chicken Soft Taco at Del Taco when you "like" them on Facebook. Hey, that works!
The Chicken Soft Taco is free with any purchase, Cinco de Mayo or no Cinco de Mayo. To get your coupon, log in to Facebook and go to Del Taco's Facebook page. Click "like" and you'll be redirected to a screen to print your one-time coupon. The coupon is valid for 10 days, mine being from May 5-15, 2010.
Want more than one? Sign up for the Del Taco Raving Fan EClub, and get two free chicken soft tacos. Plus, they'll send you a coupon for a free premium shake on your birthday, and news about other promotions. To find a Del Taco near you, visit deltaco.com/locations.html.