National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: "Impossible" Geometric Constructions
Three geometric constructions students like to say are "impossible" are explained here. Trisecting an angle, squaring a circle, and doubling a cube are examined as being either possible or impossible, depending on the "rules" you use.
University of Virginia
Basic Ideas in Greek Mathematics
This page discusses some of the mathematical problems addressed by some famous Greek mathematicians, including Zeno, Archimedes, and Eudoxus.