There were a number of things that opened my eyes. I think my first eye opener was just something someone said though. I used to be as Christian as Christian gets (I once cut a cross into my arm in order to repent before God if that's any indication of just how Christian I was). Anyways, a girl I was dating said to me something along the lines of "we all deserve to go to hell." And I guess that just set my mind to thinking about how a loving God could have a place of eternal punishment designated, not necessarily for bad people, but just for people who dissagreed with him. That question in turn lead to other questions, and ultimately not just to a rejection of my faith, but to my having come into contact with reading material which lead me to take an anti-theist position.
That said, there are a number of books and other sources that have helped me on my way. Steven Pinker's book "The Blank Slate," helped teach me about psychology, and its evolutionary roots, and actually lead me to drop what I realized in retrospect was a horrifyingly eugenic view. Works from the "new atheists," Richard Dawkins, Sam Harris, Christopher Hitchens, Bill Maher, and others, have all helped greatly in shaping my world view, teaching me to appreciate the beauty of the natural world, love my fellow humans, and also fear for our future as I realized the potential that religion holds to do great damage to our world. I would also say that reading things like "The Myth of Sisyphus" and "The Plague," by Albert Camus have given me the strength to overcome existentialist doubts that I've had throughout my life.
