Two of my old favorites are The Jupiter Theft by Donald Moffitt and Diaspora, the author whose name escapes me.
Both were the hard sci-fi books which I was aching to see come from the likes of Niven, Clarke, and Pournelle, but never saw.
If you liked The Jupiter Theft, try Moffitt’s two book series…um…The Genesis Quest and Second Genesis which I thought were even better! That’s an author who shoulda been forced (at gunpoint, if necessary 
) to produce more stuff!
Fenris
Right on, man! I’ll see if I can dig those puppies up somewhere.
Second you on that, and add Threshold, by the same man. Whatever happened to him?
I’m now home, and can check my bookshelf. Yes, T.A. Waters.
Yow. I got my copy from a bookfinder about five years ago for maybe a tenth of that. And it’s in VG condition, too.
– Bob
Ben, if you like Jeter, then you should check out Noir. It’s a cross between the pulp-noir and cyberpunk genres, and quite surreal.
As for Hard SF, I really liked Einstein’s Bridge, by John Cramer. It’s an imaginative “what if the SSC had been built?” story.
Since Trion stole my first recommendation, Illegal Aliens, I’ll have to content myself with saying, [singsong]"I’ve still got my copy, you can’t have it![/singsong] 
Other good books no one else has heard of:
Buddy Holly is Alive and Well and on Ganymede. By Bradley Denton. One day, every TV channel starts broadcasting Buddy Holly, and the signal is coming from one of the moons of Jupiter. And Buddy says that Oliver Vale knows why. Actually not half as whacky as it sounds. Half the novel is set in the present day, where Oliver is pursued across the country by government agents, raving fundies, outraged couch potatos, a cybernetic doberman, and the space aliens next door. The other half (alternating chapter to chapter) is Oliver remembering his childhood, his single mom’s obsession with rock and roll, and her steady descent into psychosis and suicide. The contrast is surprisingly effective.
Also good is Waiting for the Galactic Bus, by Parke Godwin. God and the Devil are bored, teenaged extra-terrestrials who create life on Earth as a gag. Supporting characters include John Wilkes Booth, Saul of Tarsus, and Jesus Christ. (Saul to an incognito Jesus: It was you people who crucified Our Lord! Jesus: Hey, I was against that from the beginning.) Very similar in tone to Pratchett and Gaiman’s Good Omens.
“Clay’s Ark” by Octavia Butler. An excellent novel that introduces an unusual alien lifeform into a bleak future society. Butler also authored the excellent “Exogenesis” trilogy, about an alien race that salvages the remanents of mankind because it requires a continual supply of new DNA to combine with it’s own in order to survive. Not a horror novel, although the description sounds like it could be. Butler’s style is similar to LeGuin’s, and she is a sadly neglected writer of intelligent, adult social sci-fi.
Phillip Jose Farmer’s riverworld series.
Circa sometime after the end of the world as we know it and now the entire earth is covered by on continuous torturous river. The closer you get to the beginning of the river the closer you get to the beginning of time. Also the closer you get to the huge city that seems to be the heart of the whole thing. Characters for different periods interact with each other and apparently people retain their memories from their previous lives.
Some good science fiction. I think I read about 4 books of it.
The Big Eye, Max Ehrlich. Astonishingly obscure for its quality.
Partners in Necessity, Plan B and Pilot’s Choice by Sharon Lee and Steve Miller. Also, I Dare, even though it hasn’t been published yet. 
I want to second **Fenris ** on the Aldair series by Neal Barrett Jr. I’ve still got three of the books in my possession. Oddly enough, Aldair in Albion is the one I don’t have.
A comment by CalMeacham the other day reminded me of this: If you liked the Hornblower books by C.S. Forester, you should try the John Grimes stories of A. Bertram Chandler. Especially the earlier books. The ones after his Grimes character gets married are a bit of a letdown, I think because he does a poor job of fleshing out the wife. She’s especially 2 dimensional and stereotypical, IMHO.
I can also recommend anything by Lee Killough. Excellently realized characters and good, logical story lines.
One really obscure one: The Shadow of the Ship (1983) by Robert Wilfred Franson. A human is marooned in a decidedly odd subspace universe. I don’t remember too much more about it except that I liked it a lot. Unfortunately, Franson doesn’t seem to have ever written another book.
The Lord Darcy stories by Randall Garrett. Mostly shorts, with one novel that I know of. Title character is a Sherlock Holmes type in a world that took a leeeeetle different path when Richard the Lion-Hearted survived the siege of Chaluz. Hint: the Dr Watson character is Chief Forensic Sorcerer to the Duke of Normandy.
A little closer to the topic is The Santaroga Barrier, one of Frank Herbert’s more obscure works. About a small town in California that wants nothing to do with the outside world–and why. Jaspers, anyone?
OttoDaFe: I’ll second The Santaroga Barrier . A stronger work then a lot of people give credit for, IMHO. I’m also somewhat partial to Whipping Star.
- Tamerlane
Agreed, excellent stuff!
…and I’ll add: Jerusalem Fire, R.M. Meluch.
Now there’s an odd thing… search on R.M. Meluch at Amazon, and find 7 matches (discounting repeats), all out of print, and 5 star reviews against several. 
Ok, I’ll have to warn you my instests lie more in the direction of sci-fi that blurs the line between reality and fantasy…as opposed to “that space stuff” some people prefer.
** Folk of The Air** by Peter Beagle. I don’t think this book could have been more different that the last Unicorn or The Inn Keeper’s song even if he wanted it to be.
** Kaspian Lost** by Richard Grant. A teenage boy wanders into the woods, and finds…something. He returns 3 days later, only a day older. Aliens, ghosts, angels, an evil government conspricy, it has it all.
** The Border ** by Marina Fitch. A young Mexican woman is trying to cross into the US. A spirit guides her, but its intents are far less than clear…
My favourite obscure novels are Dragon’s Egg by Robert L. Forward and Courtship Rite by Donald Kingsbury. I have others that no one else seems to have encountered (the Dancers at the End of Time series; anything by Jack Vance, etc.) but those are my favourites.
Miller, have you read The Snake Oil Wars yet?
The Lord Darcy Series was great and the follow-up books by Michael Kurland weren’t too bad. I received Replay as a birthday present years ago and still treasure it. I enjoyed the de Chancie books but they’re kinda predictable after awhile.
I didn’t like The Flying Sourcerers but I don’t quite remember why. I do remember that I thought it was boring but I also just plain disliked the story. I just could not finish any of the Riverworld books. Not engaging enough, I suppose. I also didn’t really like any of Butler’s books; she is a good writer but not my preferred style.
Joanna Russ’s The Female Man. I really really like this book. We who are About To rocked too. Her book of feminist literary criticism How to Suppress Women’s Writing is hard to find but very worth reading. It’s witty.
Robert Wilson’s Schrodinger Cat Trilogy.
Wacky and zany.
OK, I’ll make it a threesome, chiming in with Tamer and Otto to recommend Santaroga Barrier. In fact, I’ll go further and recommend more of Herbert’s lesser known novels, including Whipping Star and Dosadi Experiment, as well as Hellstrom’s Hive and the short story collection The Worlds of Frank Herbert.
Unfortunately they’re all out of print, but you should be able to find some of them in used bookstores.
And Dave, I liked Schrodinger’s Cat better than Illuminatus! even though they share a lot of material. But it never gets mentioned.