Iain M. Banks -- any of the Culture novels I like these books a lot, though they can be a workout -- his writing is very dense, and he doesn't spoon-feed you anything. The Culture is all about what a post-scarcity world would be like. I totally believe that the future might look like this, and that would be fine. Excession is probably my favorite. I need more of them, but I want mass-market paperbacks (rather than trade paperbacks) and I shop used bookstores more often than new, so my collection is incomplete. Far future.
David Brin -- Uplift novels Pretty solidly in the "space adventure" category, with a very full and complex cosmos. Thousands of alien races, dozens of ways to get around Einstein, and so on. Characters with realistic psychology in a complicated, messy universe. His other stuff is good, too. Far future.
Lois McMaster Bujold -- Miles Vorkosigan novels Okay, so there's some sort of convoluted backstory to let the setting be vaguely 19th-century, socially, but the characters are all very real people, and the plots are great, and Miles Vorkosigan is a really fun character to read about. Plus, the other planets have very different cultures and there's a lot of exploration of the differences. Probably one of my favorite authors. Mid-future (near future tech, far future history).
Arthur C. Clarke -- Venus Rising series This is actually a bunch of old Clarke short stories reworked into a sort of sci-fi masonic conspiracy story arc. It's a little weird, but it's got a lot of good "bits" (the idea cores of the original shorts) and it's sorta cinematic. Been quite a while since I last read these, so take it with a grain of salt. The framing does a lot for taking concept-based sci-fi and making it feel surrounded by a world with some depth to it. Near future with lots of tech.
Michael Flynn -- In the Country of the Blind This is an interesting one. It's basically set "today", with the premise being that there's a secret group that has discovered the laws of history, and can accurately predict the future. It's not fantastic, but there are a lot of fun ideas that he follows through to their natural conclusions.
C.S. Friedman -- This Alien Shore This is a crazy complicated novel that feels believable to me because the world it depicts is different from ours on so many fronts -- and yet, so many things are still the same. I still can't decide whether it's complete with an ambiguous ending, or whether it's book 1 of a series.
The Wild Cards series -- George R.R. Martin, editor This series takes the existence of superheroes as a given (with a suitable sci-fi excuse) and then tries to make it as realistic as possible. Written by a bunch of authors who game together. Stop after you get to book 7; they go rapidly downhill after that one. Set day before yesterday in a world three steps to the left.
Anne McCaffrey -- Dragonriders of Pern books I don't know if I really find the world realistic or if I'm just being nostalgic because I like these books a lot. They're only technically science fiction, but there's a lot of action rediscovering lost science from the past and an overall modern mindset behind it. I never got through several of the later books; it's really the first trilogy that's the best.
H. Beam Piper -- Little Fuzzy, Fuzzy Sapiens; William H. Tuning -- Fuzzy Bones Tuning's book was written before the posthumous discovery of Piper's third Fuzzy book; I think it's arguably better, but you can almost merge the two universes together. It's a future extrapolated far forward from the 50s (boy, do the characters smoke and drink a lot!) but along with some unconscious sexism and vacuum-tube computers it has a refreshingly sincere and uncynical optimism to it. The fuzzies are all about kawaii.
Kim Stanley Robinson -- Red/Green/Blue Mars This trilogy charts the colonization and terraforming of Mars from day after tomorrow to a few centuries into the future. With the way that things unfold, the politics and culture and individual personalities, they really feel like actual history that just hasn't happened yet. They made me feel like that's how it actually will happen, when we get around to it. I just wish that there had been any characters, at all, in the entire series, that would have acted even a little bit like normal sane people once in a while. (One of the points of the books is that normal people don't move to a hostile world, but still.) Very realistic characters -- I just wish I didn't want to smack them the whole time.
Melissa Scott -- Burning Bright, Dreamships, Trouble and Her Friends I first read these mostly because she has lots of queer characters that are queer as a matter of course, rather than as a defining trait. Her futures lean towards the gritty side of realistic, but there's a lot of richness to them and characters that are very human in their fallibility. Her worlds feel like they've got as much going on in them as the real world does. Burning Bright prominently features an entertainment recognizable as a descendant of tabletop RPGs, which is neat. She has a number of good fantasy novels as well, including an interesting fantasy-based spaceship trilogy. Various degrees of futureness.
Neal Stephenson -- Snow Crash (also Crypotonomicon, The Diamond Age, and, as "Stephen Bury", Interface) Snow Crash is one of my favorite books. It's fast and clever and funny and wacky, and it was the first cyberpunk novel to do something other than rip off Gibson -- not to mention the first with a sense of humor. I totally believe in his vision of the future because it is fully as crazy and stupid and peculiar as the real world. Be warned, Stephenson does have his failings -- he can't name his characters (the main character is "Hiro Protagonist", I shit you not), the scientific premise of the novel is totally broken, and the man cannot write endings -- his books just sort of come to a crashing halt about three pages after the action climaxes. But it's a great ride all the way through, and I love his writing. The Diamond Age is also very good, though set further in the future, far enough that the world is starting to get hard to understand, and the book has a very different tone. Cryptonomicon is good -- very late 90's, flashing back to WWII a lot. I haven't read the Baroque cycle, because it's freaking huge. Oh, and there's Zodiac, which is marginally sci-fi and mostly "eco-thriller" with lots of gonzo environmental science. Interface is a lot of fun, too; I think it was mostly written to bring the book, by simple and logical steps, to a situation straight off a tabloid headline, and to comment on what it would take to bring certain political eventualities to fruition. Stephenson is a lot of fun to read.
So that's a very particular slice through the books that I like. Maybe you'll find something new you like.