I hope this isn't too politically volatile, but Milton Friedman.
I don't mean this in the sense of "Oh, his views are so wonderfully rational compared to everybody else's!" Rather, he actually influenced me in how to think.
Capitalism and Freedom had a sort of pattern where he would take something conventionally considered an insoluble social problem, poke at it for a minute, and easily come up with at least one conceivably practical improvement. It would be original, but only because nobody else was bothering to poke around. (In Friedman's case, the goal was usually to refute the idea that "It's impossible to improve X without government." But the thinking style could be applied to a variety of goals. A claim of impossibility should be shaken a little bit before accepted.)
In everything I've read of his, technical and non-technical, there's a certain cheerful, reasonable attitude; he seemed to think that humans are good at problem-solving, that we actually can think and that it's worthwhile to try. That made a great personal impact on me, at a point in my life when I believed the contrary.
Light reading about 'Rationalist Heroes'.
I am not sure how useful people find having personal heroes. I would argue that they are definitely useful for children. Perhaps I haven't really grown up enough yet (growing up without a father possibly contributed), but I like to have some people in my head I label as "I wonder what would X think about this". Many times they've set me straight through their ideas. Other times I've had to reprimand them, though unfortunately they never get the memo.
One living example is Charlie Munger.
He was an early practical adopter of the cognitive biases framework, and moreover he clearly put it into context of "something to protect":
"not understanding human misjudgment was reducing my ability to help everything I loved"
(The quote is from his talk on "Misjudgment" which is worth reading on its own http://vinvesting.com/docs/munger/human_misjudgement.html)
One interesting point is that Charlie is seemingly a Christian. I have a deep suspicion that he believes that religion is valuable, for the time, as a payload delivering mechanism.
“Economic systems work better when there’s an extreme reliability ethos. And the traditional way to get a reliability ethos, at least in past generations in America, was through religion. The religions instilled guilt. … And this guilt, derived from religion, has been a huge driver of a reliability ethos, which has been very helpful to economic outcomes for man.”
Also, judge for yourself from his recommended reading list - looks like something out of an Atheist's Bookshelf.