I'll probably be doing more than one of these but here's a good first one (in no particular order), consisting of mostly "mainstream" / well known suggestions:

* Foundation (series - HUGE) - Isaac Asimov

Concept: Mankind develops a science called 'psychohistory' that allows for extremely accurate predictions of long term events based on a model of society as a whole. Series centers around the ramifications of its use.

* Ender's Game (series - growing) - Orson Scott Card

Concept: The first book, Ender's Game, centers around the highly intelligent child named Ender who is drafted into Earth's space-military school as one of many candidates in the effort to "raise" the ultimate military commander to lead Earth to victory against an alien force. Warning: There is a new series of books from Orson Scott Card nicknamed the 'Shadow' series, which revisits the events in Ender's Game from a new perspective - and radically changes your view of what happened (but in a good way).

* Neuromancer - William Gibson

Concept: This book is the seed for cyberpunk, the father of ideas like The Matrix. "Jacking in", digital souls, dying from a network attack - it's all there. You'll NEVER believe it was written over 20 years ago.

* Snow Crash - Neal Stephenson

Concept: A worthy relative of Neuromancer. It's hard to talk about everything this book covers: mind hacking, the origin of life/language, the privatization of citizenship, high-tech pizza delivery, virtual samurai, ... Definitely a fun read.

* Sphere - Michael Crichton

Concept: Read the book (which is good) first if you haven't seen the movie (which is poor) already. A ship unlike any other on Earth is discovered deep underwater. A crew is sent to investigate and they discover several things, including: an extremely intelligent computer and a mysterious device of unknown capability.

* Hyperion and Fall of Hyperion - Dan Simmons

Concept: These books were originally a single book but the publisher made him divide it in half. Get/read both at once. These books tell the story of 7 pilgrims as they journey to the planet of Hyperion, a world where unexplained monuments are slowly traveling backwards in time. A world where they will quest after a quasi-religious mythical creature known as the Shrike - chromed, 3 meters tall, razor sharp barbs on all surfaces, the walking embodiment of death. Note: There is a second pair of books, namely Endymion and Rise of Endymion. Read these only after reading the other two.

Made all my emphasis tags open. Oh well. C'est la vie.

yes yes yes to snow crash - no to Crichton!!

have you read quicksliver and the baroque stuff from Neil Stephenson?

Yes, I've got entire Baroque cycle. I have to admit it was a real bear for me to read through the whole thing :/ I enjoyed some aspects of it, but the volume of detail and prose compared to actual plot content was overwhelming. I enjoyed Cryptonomicon much more.

I'll accept your booing on Crichton. He's not amazing scifi .. but for someone just starting out, he's _very_ accessible. The reaction is probably very similar to certain Horror purists when someone mentions Stephen King.

Maybe I should have put an 'It's ok to miss this one' disclaimer on him. I've got many more suggestions for books but I wanted to keep the first list short just to get it out the door.

If you liked Snow Crash, I recommend Earth by David Brin, anything by William Gibson, ... hrm. I should put together a cyberpunk-lovers List.

I have read all four of them and absolutely loved them!!! I am probably ready to read them again, something I rarely do. Dan Simmons is an amazing author.

If you liked these, you would probably like "The Chronicles of Thomas Covenant" too. Thomas C. is a man of our time who has leprosy and even in our enlightened time is shunned by those around him, deserted by wife and alone he just shuffles through life. Then.....one day he is transported to an alternate reality where his disease is gone (but not the habits that go with it) and, although he does not want to, joins the fight against a creeping evil that is killing the land. He is a reluctant hero and not really a very nice guy which is one of the things that make this series so good.