Shamelessly coopting this thread for an expanded /lit/ list ripped from Wired:
The Hitchhiker's Guide to the Galaxy: The book that has had the most profound influence on my life. I'm not saying that's a good thing, it's just a fact.
Ender's Game: Despite its many sequels and retellings, this book stands by itself as a profound look at the struggles of growing up different.
A Scanner Darkly: It's difficult to pick just one of Dick's novels (really, this whole list could be devoted to him), but this novel is the height of paranoid fiction.
Neuromancer: Gibson may not have invented cyberspace, but he is its greatest prophet.
Snow Crash: With a character name like Hiro Protagonist how can you go wrong?
Fahrenheit 451: My question now is, "How do you burn electronic books?"
Watchmen: Not only one of the great graphic novels, but one of the great novels of our time.
1984: Orwell's book still holds as a solid analysis of how to rob a human of any hope.
The Handmaid's Tale: I fear sometimes that there are people who would welcome this world as portrayed by Margret Atwood, and that's what scares me the most.
Vurt: Although not widely known in the US, this British import is a classic in my collection.