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.
World History Encyclopedia
World History Encyclopedia: Greek Mathematics
Discusses the major cultural influences on the development of mathematics in ancient Greece, and how the Greeks went on to surpass those cultures. The work of the Pythagoreans and of Euclid are described, along with some famous...
Rice University
Rice University: Cynthia Lanius: History of Geometry
Different cultures' contributions to Geometry are discussed.