As I mentioned in my previous blog post, a newly published anthology of ghost-themed YA stories that I participated in, Spectral Tales, is free from these retailers:
Maybe ghosts aren't your thing? How about fairy tale re-tellings? Then check out Through a Tangled Wood.
Perhaps you'd like to read a YA short story collection centered around comets? Then Celestial is for you!
I should also mention that two of my own books are free today as well!