0!

http://mathworld.wolfram.com/Factorial.html

I remember learning some other complex proof of this once, but I haven't done math in a while, so excuse my forgetfulness.

Mathworld is always a good resource, and does answer this question -- by definition. n! represents the number of possible combinations (where order matters) that n units may be arranged. 1! is obviously 1 because there is only 1 way to arrange 1 number. One could argue that there are 0 ways to arrange 0 numbers...but the arbitrary standard interpretation of this is that there is 1 way to arrange 0 numbers.

Ah! Thanks for that, you both!