Wolfram Research
Wolfram Math World: Euclid's Algorithm
This site from Mathworld.com provides a very detailed description of Euclid's algorithm of common divisors and a method of finding the greatest.
Calculator Soup
Calculator Soup: Greatest Common Factor Gcf Calculator
Calculate GCF, GCD, and HCF of a set of two or more numbers and see the work using factorization. An interactive calculator, explanations, and examples are included.