The Good News About Injustice has definitely changed the way I see things and my life in general. It's caused me to take a completely different path in life, and shown me how we really can (and must) fight evil and change the world. With any luck, I'll even get to work for International Justice Mission some day.
To a lesser extent, Kingdom Come also was an eye opener, but really it was confirming what I already suspected, and it suffered from the author's failure to escape from the false dichotomy between individualism and collectivism.
The Skeptical Environmentalist also deserves a nod, as it shows quite elegantly the problem with the lack of critical thinking that is all too common.