I've arrived too late to this thread it seems, all the books and authors I was going to recommend have already been mentioned!
Therefore, I'd like to re-recommend:
*Heinlein's The Moon is a Harsh Mistress; definitely one of the best classic sci-fis ever written. Also try Have Spacesuit, Will Travel. Some of his other books are a bit sexist and/or pretentious in this day and age, but many are well worth a read.
*Asimov; pretty much anything he ever wrote, at least of the older stuff. Nightfall, the original Foundation trilogy, his robot short stories and so on. You CAN'T go wrong there!
* Ursula K. LeGuin; earthsea is an easy choice. Very good books, except perhaps the last one or two. I seem to remember the series got a little stale in the end, but first three are very good and very unique books. Only read one other title by her, and that one was also very very good, but I can't remember the name.
* Herbert; Dune's a great, great work of literary art and a classic. Must read. Later on the series takes a huge left turn IMO, and some may or may not like them.
* Hamilton; Night's Dawn Trilogy, Fallen Dragon, the Commonwealth Saga. All worth their weight in...well, lira or pesos perhaps, hehe! But, you know what I mean!
*King/Strauss: The Talisman. Authors, but not this book mentioned so far afaik, so whee for me! It's kind of old now, so perhaps hard to find, I'm not sure if it's in print or not anymore.
*Jordan; BLECK! NO!!!!