John Brunner. Especially "The Shockwave Rider" (the book that gave us biological terms applied to computer programs, like "virus" and "worm.")
Heinlein's "The Moon is a Harsh Mistress."
Simak's "Way Station"
Niven's "Protector." Other Niven as well; by himself Niven does a great job of extrapolating possible effects of technology. With Pournelle, he's different but still good.
James Hogan had a nice take on "aware" computers in "the Two Faces of Tomorrow", and Sawyer in his "WWW:Wake" series.
For more modern authors, I like Lois Bujold (but she's a world builder, so you have to read several books before you realize how wonderfully things are put together.), Varley, Vinge (hmm. Both of them, actually. But Vernor is the "hard sf" writer), Walter Jon Williams, and Terry Pratchett...