Charles C. Mann
I heard Charles C. Mann speak at the George Wright Society Conference this spring and it completely changed the way I think about human history.
The pre-european western-hemisphere population size, sophistication, and magnitude of landscape alteration were all much more massive than I ever fathomed.
There are a couple interviews with him here and here. At the least the first link does a good job highlighting some of the interesting points he conveys in his book 1491.
This concurs with a recent discovery of mine: history is actually interesting and relevant! Its not all about dry names and dates and arbitrary discoveries and founding fathers; it's about hate and fear and sex and manipulation and lust and luck and power. Not exactly components of traditional public school curricula.
Image is from National Geographic