Cut the Knot
Cut the Knot!: Pythagoras' Theorem
This site explains the Pythagorean theorem, giving a proof from Euclid, and gives a great interactive visual aid with a good explanation with it.
University of British Columbia
University of British Columbia: Proof of the Pythagorean Theorem
A geometric model that proves the Pythagorean Theorem in a step-by-step diagram that students can advance at their own pace. Requires Java.
Curated OER
Math Open Reference: Biography of Pythagoras
Biography of Greek mathematician, Pythagoras, who lived from 569 BC to approximately 475 BC. His theory, known as the Pythagorean Theorem continues to be of interest to mathematicians and has more than 400 different proofs.