Here are the most useful links with answers why does zero factorial equal one, i.e. 0! = 1:

http://www.quora.com/Mathematics/Why-does-zero-factorial-0-equal-one

http://wiki.answers.com/Q/Why_is_zero_factorial_equal_to_one

http://en.wikipedia.org/wiki/Factorial

http://math.stackexchange.com/questions/25333/why-does-0-1

https://ca.answers.yahoo.com/question/index?qid=20090116132324AACQIGU

http://www.zero-factorial.com/whatis.html

http://statistics.about.com/od/ProbHelpandTutorials/a/Why-Does-Zero-Factorial-Equal-One.htm

http://mathforum.org/library/drmath/view/57128.html

[factorial, factoriel, factorial function, zero factorial]

http://www.quora.com/Mathematics/Why-does-zero-factorial-0-equal-one

http://wiki.answers.com/Q/Why_is_zero_factorial_equal_to_one

http://en.wikipedia.org/wiki/Factorial

http://math.stackexchange.com/questions/25333/why-does-0-1

https://ca.answers.yahoo.com/question/index?qid=20090116132324AACQIGU

http://www.zero-factorial.com/whatis.html

http://statistics.about.com/od/ProbHelpandTutorials/a/Why-Does-Zero-Factorial-Equal-One.htm

http://mathforum.org/library/drmath/view/57128.html

[factorial, factoriel, factorial function, zero factorial]