Plenty of good SF over at Fictionwise. They've got stuff by writers like Greg Bear, Brian Stableford, George Alec Effinger, short stories by Nancy Kress, Mike Resnick, older stuff by Pamela Sargent and James Tiptree Jr., etc.
I suggest browsing the catalogues of
e-Reads,
Wildside Press,
Small Beer Press,
Wonder Audio Books (if you like older Golden Age stuff),
ElectricStory, and maybe the
Marion Zimmer Bradley Literary Works Trust (if you like her work and that of her protégées, though it's mainly fantasy).
Plus you can get subscriptions for Analog and Asimov's magazines, along with a couple of others.
My very highest recommendation goes to Ted Chiang's
Stories of Your Life and Others. My second highest recommendation goes to Terry Bisson's
Bears Discover Fire. My third highest recommendation goes to George Alec Effinger's
Budayeen Nights, which loses a couple of points for annoying formatting issues.
After that, it gets a bit iffy because it looks like Fictionwise has been removing MultiFormat stuff from their catalogue with no good reason again, plus I'd have to know more about your reading tastes before I could really recommend any other lesser stuff.
Hope this helps.