Links

Videos

Counting Using Permutations (time: 10:29)
http://www.youtube.com/watch?v=-mC_QK6dBIY

Permutations (time: 10:10)
http://www.youtube.com/watch?v=XqQTXW7XfYA

History

This is one of the best all around sites that I have found. It is titled "The MacTutor History of Mathematics Archive."
http://www-history.mcs.st-and.ac.uk/~history

This is an award-winning site that links to all sorts of mathematics information.
http://archives.math.utk.edu/

Reference Topics

Discusses the building of tree diagrams.
http://www.shodor.org/interactivate/discussions/pd8.html

Permutations:
http://www.cut-the-knot.org/do_you_know/permutation.shtml

Permutation patterns:
http://www.blarg.net/~math/permutations.html