Okay...I had to think about this for a while, and ultimately decided to include only books that changed my life, as in very significantly shaped the person that I am today. With that in mind:
Stonewords: A Ghost Story by Pam Conrad
This is the first book I ever remember being completely enthralled with. I remember staying up all night to read it cover-to-cover and being chilled and saddened both by the fate of the main character's best friend. I would count this and Susan Cooper's The Dark is Rising as the most influential books in my childhood.
Jane Eyre by Charlotte Bronte
Reading Jane Eyre in middle school marks the first time I was truly enchanted by a fictional romance. I remember being in chills and crying while reading Jane's breakdown ("Do you think, because I am poor, obscure, plain, and little, I am soulless and heartless? You think wrong! I have as much soul as you, and full as much heart! And if God had gifted me with some beauty, and much wealth, I should have made it as hard for you to leave me, as it is now for me to leave you.") and Mr. Rochester's proposal. A lifelong love of fictional romances ensued.
Harry Potter and the Sorcerer's Stone by J.K. Rowling
This series will always hold a special place in my heart, but I consider it "life-changing" primarily because my interest in the Harry Potter books got me involved in internet culture. I met several very close friends through Harry Potter message boards and chatrooms. I still think that Ms. Rowling crafted some of the best-plotted books I've ever read, whether intended for children or adults.
The Great Gatsby by F. Scott Fitzgerald
The excesses, the tragedy...
1984 by George Orwell
I think the thing I most took from reading 1984 in high school was an appreciation for the power of written and spoken language: particularly the idea that restricting our way of expressing thoughts actually restricts the way we think. I was fascinated by the way that society phased out dissent and the younger generation had no recollection of what it was like to be free. The ending is also very affecting.
The Road by Cormac McCarthy
I read this in college, and I've rarely felt such an instinctive connection with a novel. It contrasts the fragility of human life with the enduring power of familial love. It's one of the only books that has made me cry every time I've read it. Reading this book shifted, in some small but significant way, the way I view people in my life and value connecting with people.