Easily, one of my favorite books ever. I have bought, read and shared it with others several times already. It influenced my political thought, turning me (even more) towards anarchism. I found this novel was an excellent exploration of (a particular implementation of) an anarchist society, how it could prosper, what problems could arise (e.g. individualism vs. collectivism).

It even influenced my personal feelings, how I relate to others in my daily life, as the story is written with a lot of consideration to personal and emotional details. Sharing some of the quotes that touched me most deeply was the main motivation for me to start using sigs in my mail client of choice (mutt). Here are some of them:

"-Mine! Mine sun!
-It is not yours. Nothing is yours. It is to use. It is to share. If you will not share it you cannot use it."

"Well, of course. If you don't want to do the work, you should not do it."

"The duty of the individual is to accept no rule, to be the initiator of his own acts, to be responsible."