Free digital copy of the first issue of Injustice: Gods Among Us, the official tie-in comic series to the upcoming game. Requires an account, though I was able to log-in with my Comixology account.
There are other digital comic books on discount too like the first six issue of the New 52 JL, and older versions of JLA.
You might like to put the word comic somewhere in the title or other people might think it's a free copy of the game instead