Others have mentioned most of them, but:
Atlas Shrugged and / or The Fountainhead -- AS is generally considered one of the greatest English novels of all time.
Heart of Darkness by Conrad
One of Primo Levi's books -- He was a Holocaust survivor and an incredible writer.
Don Quixote -- Like someone said, it is considered one of the best books ever written.
The Skeptical Environmentalist -- interesting perspective considering where we've wound up today.
A Short History of Nearly Everything by Bill Bryson -- After reading it, you realize how much science textbooks suck at being interesting.
The Brothers Karamozov -- Long, but a great read.
A Tale of Two Cities -- it's on your list, but I just wanted to reinforce that you should read it.
The Great Gatsby
Great Expectations
Slaughterhouse Five
100 Years of Solitude
The Bible or Paradise Lost -- both cover essentially the same ground, but Paradise Lost does it with more style.
The Old Man in The Sea
1984
Animal Farm
Ulysses by James Joyce
The Sun Also Rises by Hemingway
One Flew over the cuckoo's nest by Ken Kesey
Ishmael -- I dislike this book thoroughly, but I think people should read it and make up their own mind.
War is a Force That Gives Us Meaning -- Yes, a little hippey, but it makes you think.
Siddhartha by Hermann Hesse
The Communist Manifesto -- again, something most people won't agree with, but an interesting read nonetheless.
The Prince
Leviathan (at least excerpts)
The Social Contract
Two Treatises of Government and A Letter Concerning Toleration by John Locke
The Catcher in the Rye by J.D. Salinger -- Another book I can't stand, but one that you should form your own opinion of.
The Meditations by Marcus Aurelius
Where I'm Calling From by Raymond Carver
To Have or to Be? by Erich Fromm
Tuesday's with Morrie
I'm sure I can think of others.
The Odyssey