Publishers and multi-genre sites go here.
Belgrave House (mystery, romance, miscellaneous genre fiction). Offer some free mystery and romance short stories and a novel from their
Belgrave House main site and some more romance novellas via their
Regency Reads imprint. Reprint many backlist authors' books which were previously published with romance and crime imprints (generally tends to more cozy/women's fiction mysteries, but some historicals and harder crime in there as well). Couponable at Fictionwise, Kobo, etc.
Book View Café (consortium of backlist authors, mostly fantasy & science fiction, but some historical, romance, mystery, other). Offers an occasionally rotating selection of
downloadable freebies on their website (currently short stories) and host free online-read short stories & novellas from their authors. Rarely if ever hold sales, but generally very reasonable pricing at mostly $2.99-$4.99 for individual novels, with online reading access and multiple DRM-free formats. Some titles also re-sold via other outlets and may be couponable.
ETA: Huh. Apparently they've removed almost all the short fiction online reads from the site. Too bad, because it was a good promo thing that I liked pointing people towards when suggesting they pick up books by particular authors. Hopefully they're just temporarily down as part of a planned site reorganization since their site seems to now be severely functionally broken in other basic ways, but I'm cynical and suspect they've disappeared for good.
Evernight Publishing (newbie imprint, romance & erotic romance). Has a small
free reads section on their webstore which they seem to add to sporadically, and offers a rotating
weekly 99 cent discount feature title which you can keep up with via their blog (they don't tag the entries meaningfully, but they seem to change it every Friday).
Infinity Plus (science fiction, fantasy, speculative literary, some mystery/crime @ $0.99-$2.99
via Smashwords). Do mainly short story collections but also have backlist reprints of established, often award-nominated authors. Offer a free short story and a freebie sampler containing full-length short stories and a novelette from their published authors:
One Step Closer,
Infinities
MLR Press (m/m romance and LGBT detective/other subgenre fiction; have also started an f/m romance spinoff imprint) offers a free PDF download of one of their titles every couple of weeks on a rotating basis,
downloadable from the top of their site's frontpage. Couponable at Fictionwise, Kobo, etc.
Night Shade Books (fantasy, horror, science fiction), offer a couple of downloadable short stories from their sometimes award-winning authors
on their website. Couponable via Kobo and others, but generally cheapest bought
via Baen's webstore at $6 DRM-free MultiFormat vs $7.99 pre-coupon most other outlets. Occasionally hold special discount pricing and give out freebies to Amazon.
Samhain Publishing (romance, erotic romance, horror) offer a couple of permafreebies in their webstore which I'll dig up later once I figure out which ones they are. They also have the old Samhellion author promotion blog at which you can get
free PDF short stories by their authors which often tie into their on-sale series. Offer 30% off new release and pre-order titles each week and run coupon sales every so often, usually during holidays, which stack on top of the 30% weekly discount as well. Coupons generally 1-use, so stock up your cart before checking out.
Small Beer Press (science fiction, fantasy, speculative literary fiction). Offer a selection of downloadable works under Creative Commons license for promotional purposes
on their website. You may also find extra stories in the samples and podcasts sections. Couponable via Fictionwise, and sometimes offer discount sales in their own webstore (special event holidays, IIRC).
I especially recommend Maureen F. McHugh's story collection, which is free and which I happen to own in paper as well, and the Ted Chiang collection (not free, but well worth the money, and also a paper-own for me in two languages).