1. The HitchHiker's Guide to the Galaxy -- Douglas Adams 85% (102)
Read it. Enjoyed it. Quality of series dropped steeply after the second book, so don't feel bad if you find yourself let down by the sequels.
2. Nineteen Eighty-Four -- George Orwell 79% (92)
A big classic. The most influential book I can think of.
3. Brave New World -- Aldous Huxley 69% (77)
I read it back in high school. Biggest take-home I got from it was that it familiarized me with the Falkland Islands (over which Great Britain went to war with Argentina for about 15 minutes several years ago) because the main character wanted to find the biggest hellhole on the planet, and the Falklands were it.
4. Do Androids Dream of Electric Sheep? -- Philip Dick 64% (67)
Never read it. I probably should.
5. Neuromancer -- William Gibson 59% (66)
Read it. Didn't take away much from it.
6. Dune -- Frank Herbert 53% (54)
Good but very very dense. Reads better nowadays because you have Google for when you realize that you have no idea what the Ritual of Gnar'th Bnoth Haddlebleganoschnook is.
The 1980's movie was laughable. So was the miniseries. The novel is unfilmable. Live with it.
7. I, Robot -- Isaac Asimov 52% (54)
A classic set of short stories. Way before their time and very influential. Movie adaptation took nothing from the series but the character's names, so don't bother with that.
8. Foundation -- Isaac Asimov 47% (47)
Probably the first "deep" sci-fi novel I can recall. There's no real action or technology to speak of. Just the politics of 10,000 years from now when the galaxy is (presumably) fully populated.
This series is also unfilmable, as the story takes place over thousands of years and there is no action (save for a couple of sequences with The Mule) beyond politics. Of course, that won't keep Hollywood from trying to do it.
Perhaps starring Keanu Reeves. Yeah, that'd rule.
9. The Colour of Magic -- Terry Pratchett 46% (46)
Read and enjoyed. You'll want to read the sequel The Light Fantastic too, as it ties up all of the loose ends of the first story.
Only complaint is that the satire is a bit too British and makes fun of stuff that just doesn't play in the states. I more enjoyed Pratchett's Bromiliad Trilogy for satire.
10. Microserfs -- Douglas Coupland 43% (44)
Finally, a book that I truly couldn't stand. No plot at all. Impossible situations. Corny unlikeable characters. There's nothing at all to recommend about this mess.
If you want to read a good treatment of computer-geek culture, read Hackers by Steven Levy. It's got two advantages over Microserfs. One is that it's not written by a hack who can get damn near any stream-of-consciousness crap published. Two is that it actually happened.
11. Snow Crash -- Neal Stephenson 37% (37)
Read it. Enjoyed it. Seemed to go in several different directions (virtual reality, floating island-nations, eskimo supervillains), but it was well done.
12. Watchmen -- Alan Moore & Dave Gibbons 38% (37)
Never heard of it.
13. Cryptonomicon -- Neal Stephenson 36% (36)
I got about 200 pages into it before I gave up on its nearly impossible length.
14. Consider Phlebas -- Iain M Banks 34% (35)
Never heard of it.
15. Stranger in a Strange Land -- Robert Heinlein 33% (33)
I read about half of it. I really enjoyed the setup and characters, but it seemed to be all about characters and setup and not about story.
16. The Man in the High Castle -- Philip K Dick 34% (32)
Never heard of it.
17. American Gods -- Neil Gaiman 31% (29)
Heard of it. Haven't read it.
18. The Diamond Age -- Neal Stephenson 27% (27)
Never heard of it. Lotsa Neal Stephenson in this list.
19. The Illuminatus! Trilogy -- Robert Shea & Robert Anton Wilson 23% (21)
Read it. Enjoyed it, but only mildly.
20. Trouble with Lichen - John Wyndham 21% (19)
Haven't read it. I probably should.