Here is a list of the books that affected me in one way or another, and that I'd highly recommend. In no particular order, with running personal commentary, sorta.
Dune - Frank Herbert (mentioned before, it's good enough to mention again)
The Elenium Trilogy - David Eddings (my first "serious" fantasy series I ever read. deeply affected my world view and completely reshaped my imagination)
The Mars Trilogy - Red Mars, Green Mars, Blue Mars - Kim Stanley Robinson (my all-time favourite sci-fi novels ever. incredibly written, immaculately detailed, phenomenal story - it has depth, it has texture, it's political, it's religious, it's commentary on human-kind... Blue Mars just might be my favourite book ever.)
The Hitchhiker's Guide to the Galaxy (and the four subsequent novels) - Douglas Adams (surprised nobody's mentioned this one yet. brilliantly witty, smart and meaningful. short enough to read in an afternoon, but wonderful enough to reread a thousand times.)
Ender's Game - Orson Scott Card (another brilliant sci-fi novel. well-known in popular culture, too (xkcd recently did a strip on it). i've tried to read the other books in the series, but they go off on weird religious tangents (kinda like the other Dune books). stick to this one.)
Starship Troopers - Robert A. Heinlein (great book. way different than the movie. definite American Cold War under- (and over-) tones.)
Consider Phlebas - Iain M. Banks (the only Banks book I've read. it's not really a happy book, but some great technology and super-future sci-fi writing.)
1984 - George Orwell (i read this for the first time on the beaches of Dubai this April. one of those books i always intended to read but never got around to it. i get into reading moods where i feel i should read books that i should have read and this was one of my selections)
Neuromancer - William Gibson (another sci-fi book, revolutionized the genre, etc. & etc.)
Naked - David Sedaris (amazingly hilarious book. pick up anything that Sedaris has written - you won't be disappointed. semi-autobiographical.)
The Human Stain and American Pastoral - Philip Roth (two amazing books by an incredible writer. difficult reads, but deserving of all the praise they got.)
Love in the Time of Cholera - Gabriel Garcia Marquez (i'll admit i read this book (and the next) only because Rob Gordon says that he's read them. however, it is an incredible tale that my feeble grasp of the english language can't do justice)
The Unbearable Lightness of Being - Milan Kundera (see above)
A Heartbreaking Work of Staggering Genius - David Eggers (a style reminiscent of David Sedaris but a story wholly his own - mostly autobiographical, exceptionally funny and altogether a great read)
The Zombie Survival Guide - Max Brooks (READ THIS BOOK TO SURVIVE THE ZOMBIE APOCALYPSE. DON'T BITCH TO ME WHEN YOUR SAFE HOUSE IS OVERRUN.)
Che Guevara: A Revolutionary Life - Jon Lee Anderson (i was never into the whole revolutionary thing, that whole faux-scene or whatever you want to call it, but i've always been interested in Che and who he was. this is de facto the greatest biography of him ever written. impeccably researched, it portrays him as non-biased as possible. he lead an incredible life, whether you agree with who he was or not.)
House of Leaves - Mark Z. Danielewski (this book mindfucks you. entertaining, scary as hell, like no book you've ever read. if you want to experience something literary that's indescribable, read this book.)
There's my list. Enjoy, please.
