Wolfram Research
Wolfram Math World: Euclid's Postulates
This site from MathWorld.com provides a listing of the postulates of Euclid and a brief discussion on the validity of the fifth postulate.
TED Talks
Ted: Ted Ed: Euclid's Puzzling Parallel Postulate
Euclid, known as the "Father of Geometry," developed several of modern geometry's most enduring theorems--but what can we make of his mysterious fifth postulate, the parallel postulate? Jeff Dekofsky shows us how mathematical minds have...