Twelve Hidden Days of Christmas, by Macky Pamintuan, seems to be the first freebie NOOKcolor book, courtesy of secular publisher Sterling.
This is a "Nook kids read to me" book and a notice on the page says that it works exclusively on NOOKcolor. Nevertheless, non-NOOKcolor owners can still "purchase" and presumably download it for future use.
Synopsis
Kids love searching for Christmas gifts, and they can get a head start on the holiday with this joyful twist on the popular carol. You'll recognize most of the words from "The Twelve Days of Christmas," but instead of "my true love" giving the presents, jolly Santa's leaving them all around the house. Sharp-eyed children can accompany a curious little bear to look for St. Nick's cheerful handiwork-as well as some festive extras that artist Macky Pamintuan has playfully added to the art, including jingle bells, a candy cane in the toothbrush cup, and bright snowflakes floating over the bathtub. As the items accumulate, the delight heightens, with ten frog lords a-leaping and sliding; nine pretty pink pig ladies dancing a ballet; and cow maids ready to do their milking. The best is saved for last in a big ending: a huge double gatefold that's actually four pages long, containing all the items delivered during the twelve days.
Also a large number of Christian Fiction freebies have turned up and currently seem to be in the process of being individually posted. A summary listing of their titles can be found via the very useful
Free NOOKbooks thread on the B&N boards (scroll down to the latest updates for the ones which are still free).