I'm a little late replying to this thread but here goes,
Some of my favorite authors/books which haven't been mentioned:
Robert J. Sawyer: The Terminal Experiment, Hominids, Humans, Factoring Humanity
Robert Sawyer seems to have a very good grasp of science. His use of hard science blended with philosophical/moral issues make for excellent reading.
Eric Nylund: Signal to Noise and A Signal Shattered (sequel)
These are the only two novels I have read by Nylund but they are excellent. I've heard them referred to as 'hyperpunk' but I'm not sure what that means. It is, IMHO, an excellent story.
Review
Some other previously mentioned works I like:
Arthur C. Clark: Rama series, A Space Odyssey
Isaac Asimov: Foundation series, Robot series
My favorite fantasy series:
Stephen R. Donaldson: The Chronicles of Thomas Covenant, the Unbeliever (had to mention it)