I don't read horror so I probably wouldn't recognize most of the names. But here are the people at Smashwords who've had official freebies from their publishers or been mentioned by other posters or put their credentials straight in the blurb.
Bram Stoker award-winner
John Everson (
ISFDB) offers a holiday-themed short story collection free with coupon and 75% off a collection of "magic, horror, dark fantasy, and the macabre" including an Honorable Mention story listed in the Year's Best Fantasy and Horror anthology series.
Bram Stoker finalist
W.D. Gagliani (
ISFDB) offers several shorts and a collection free to download or free with coupon.
Carole Gill (
ISFDB) offers a novel at 50% off. Her publisher
VampLit also seems to offer 50% off some other authors' works, and there's also a free-to-download novella in there as well.
Dragonlance tie-in novel writer
Stephen D. Sullivan (
ISFDB) offers selected works at 25% off.
Buffy and other tie-in novel writer
Jeff Mariotte (
ISFDB) offers a short free with coupon and other works at 50% off.
Imajinn-published Canadian
Cheryl Kaye Tardif (
ISFDB) offers 25% off a short story collection.
David Bain (
ISFDB) has a permafreebie short story and offers 50% off his Grindhouse collection.
Aaron Polson (
ISFDB entry) offers several free to download stories and 50% off his Loathsome, Dark, and Deep collection.
Melissa L. Webb (
ISFDB) offers several short stories free to download and several more free with coupon.