Euclid (circa 300 B.C.E.) is most famous for his book Elements, which for a long time was the book on geometry, and remains a classic and useful reference. It has also had a large influence on mathematics in general, because it was one of the first attempts at formalism. While the original book was destroyed, several translations are available in libraries, and copies of those translations are available in multiple forms on the web, including with interactive applets to illustrate the theorems.
Sites should be about Euclid or his Elements (for example), not just about Euclidean geometry.