White Teeth, Zadie Smith

Fierce Invalids Home from Hot Climates, Tom Robbins

Homage to Catalonia, George Orwell

Catch 22, Joseph Heller

Timequake; Mother Night, Kurt Vonnegut Jr.

Seven Pillars of Wisdom, TE Lawrence

The Histories, Tacitus

The Catcher in the Rye, JD Salinger (caveat, it was most effective when I read it at 16)

Redwall, Brian Jaques (got me really excited about reading when I was in the fourth grade)

This list could be much, much longer, but those are the most prominent 'life-changers' that come to mind.