Kopi af Einstein's Proof of the Pythagorean Theorem
Steven Strogatz has a pretty piece at the New Yorker http://www.newyorker.com/tech/elements/einsteins-first-proof-pythagorean-theorem about a proof of the Pythagorean Theorem which Albert Einstein did as a boy. It's quite nice, and really gets at the reason why the Pythagorean Theorem is true. It even generalizes nicely.
More GeoGebra at bit.ly/mh-ggb