Well, no backlist freebie sf/fantasy yet as far as I can tell in the daily KDP select slushpile, but there's a bonanza of mystery/horror thriller novels and short stories from much-discussed pro-to-self-pub author J.A. Konrath/Jack Kilborn and frequent co-writer Blake Crouch.
At least 30 of their singly and jointly-written self-pub titles have shown up as free on Amazon as part of the exclusive-or-else program, and many of them seem to be DRM-free, for those with other readers who want to give their writing a try.
I have no idea whether this is a planned promotional stunt for the holidays or just a massive glitch when setting the exclusivity settings, but if you want these, pick them up post-haste before they go and change things for whatever reason.
There's no way I'm typing out and linking this many titles, so I will simply provide you with Amazon's "works by this author" links, pre-sorted low to high. Be sure to trawl through all three names, as while there's a good deal of overlap, a couple of the books seem only to be listed "solo".
There's even a freebied translation of one of the freebied thrillers into German, if you want to try out your language learning skills.
Writing as J.A. Konrath: in Amazon
main,
UK,
DE,
ES,
FR,
IT.
Writing as Jack Kilborn: in Amazon
main,
UK,
DE,
ES,
FR,
IT.
Written by Blake Crouch: in Amazon
main,
UK,
DE,
ES,
FR,
IT.
Happy reading, if you happen to like mystery/horror thrillers.