Wolfram Research
Wolfram Math World: Permutation
This MathWorld page gives information about permutations. Its real merit lies in the fact that there are examples for a clear understanding at various parts of the site and many links to learn detailed information about various aspects...
Cut the Knot
Cut the Knot!: Permutations
This page defines permutations, offering both mathematical definitions and examples and a practical explanation of how permutations are used.