Links

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

Here is a comparison of permutations and combinations.
http://www.mathsisfun.com/combinatorics/combinations-permutations.html

A review of counting principles including the fundamental counting principle, permutations, combinations, and distinguishable permutations.
This is the elementary version:
http://www.mathagonyaunt.co.uk/STATISTICS/ESP/Perms_combs.html
This is the slightly more advanced version:
http://www.richland.cc.il.us/james/lecture/m116/sequences/counting.html

This site shows a table contrasting the basic counting types of problems.
http://www.math.hawaii.edu/~ramsey/Counting.html

This (somewhat advanced) site is very good if you take some time to understand how to use it. By selecting the second object icon (at the top of the page), you go to an index page. From this index you can find information on permutations, combinations, or distinguishable permutations.
http://www.theory.csc.uvic.ca/~cos/root.html