Andre Norton. Possibly the best "hard" sci-fi author of all time.

And by hard sci-fi, I probably mean "classic" or whatever.

Here's my answer, tilted towards stuff that hadn't been mentioned yet (so I left out LeGuin & Lessing, two of my favorites) and without links, which didn't copy over and I'm too lazy to replicate.

Elizabeth Hand. I've read just about everything she's ever written and I adore her. On preview, drat you, aperture_priority!

Also, nthing Connie Willis, and has anybody mentioned Kage Baker? Time travel, evil corporations and good, tight, funny writing. Jo Walton, for fantasy. Katherine Kerr, who has written a couple of very good sci fi books as well as her fantasy stuff. And, Wilhelmina Baird! Her totally awesome cyberpunk trilogy is right there on the front page of that link.

i guess diana gabaldon's outlander series might qualify as sf to some, but with additional elements of historical fiction and romance thrown in for good measure.

Marion Zimmer Bradley (The Mists of Avalon is a favourite)

Connie Willis - Doomsday Book is a must, To Say Nothing of the Dog is very good too.

Kage Baker - I've read two of them, need moar!

Maria Doria Russell - The Sparrow is awesome.

Jacqueline Carey - I've read the first three books of Kushiel's Legacy and they're good; alternative history/fantasy with sexy sex.

Diana Gabaldon - The Outlander series is good; I'd label it fantasy/historical fiction. Some people would label it romance and it does have elements of that, but it's well written and she's obviously done her history research.

I'll give another yay to Angela Carter (link to extensive essay on her writings on Scriptorium), who was mentioned in the MeFi thread... I like the text and subtext of her stories. For those of you who remember "The Company of Wolves" - she wrote that. The fairy tale as as an exploration into sexuality, and the subtext (and perhaps even tangential sub-texts, if there's a word for that) of that film bear repeated watchings. A note on the gore in the film - early fairy tales were harsh morality tales, and I'd think that Ms. Carter riffed off of the first of the original tales, before they were cleaned up by Charles Perrault.
Little girls, this seems to say / Never stop upon the way / Never trust a stranger friend / No-one knows where it may end / As you're pretty, so be wise / Wolves may lurk in every guise / Now as then, 'tis simple truth / Sweetest tongue has sharpest tooth.

Karen Finley. She's constantly labeled as a performance artist, but at one time I did have a collection of her writings... no wait - they were a shared thing - a friend of mine has got 'em. They're good. Powerful.

er. ok, so ms. findley isn't sci-fi, i still can't recommend her enough.

Oh Zack_Replica! I don't like sci-fi, I was just bored and checked this thread, and oh, man, how much do I love Karen Finley. I'm lucky enough to have seen her live. And you're a youngun, Zack, right? So gratifying.

Young? Not me. I just look it. I remember when U2 released "War" and when The Pogues put out "Rum, Sodomy and the Lash" and I saw The Cult tour for their "Love" album. I was almost out of high school at the time.
Would've love to see Karen Finley, though. Bet she kicked ass.