According to a Nook FAQ, Barnes & Noble directly interfaces with Google Books.
How do I get free public domain titles from Google Books?
Free titles are indicated by a "Get Free" button in place of the "Buy Now" button. Barnes & Noble.com has partnered with Google Books to bring you half a million public domain titles for free. Look for these titles throughout the website.
I know i can search for free books by putting in 0.00 into the search bar, but that turns up a bunch of crappy romance/erotic books I don't want. Does anyone have a successful way to find the free Google classics?