Most fairy tales are.
Anything by mother goose, Brothers Grimm, etc.
(E.g. Little Red Riding hood, sleeping Beauty, Puss in boots, Blue Beard, Cinderella -- all from the late 1600's. Snow white is from the early 1800's.
The one thing to remember, is that while the *originals* are often public domain, modern adaptations are not. So while you can do your own version of a snowwhite story, be careful not to include any of the changes added by Walt Disney or you're technically violating their copyright.
(I have no idea if this is the case or not, but I could imagine that the names of the dwarfs in the disney cartoon are different from the original story)
Some other old works from the public domain:
H.G. Well's "Time Machine" (1895), "The Invisible Man: (1897) and "War of the Worlds" (1898)
Mary Shelley's "Frankenstein" (1818)
You can find a bunch of public domain books on this site:
http://www.authorama.com/do note that the original public domain versions of some of these stories can be wildly different from modern adaptations (many of the fairy tales have much more gruesome endings in the original versions, and the "Time Machine" movie had very little little in common with the original book, just to mention two examples. If you're interested in doing a game about one of these books/stories, make sure to actually read the original version and try not to let yourself be influenced too much with other info you may have picked up about them over the years, because those modifications likely will be copyrighted by someone else)