"Here is wisdom.  Let him that hath understanding count the number of the beast: for it is the number of man; and his number is Six hundred three score and six." 

The "beast number"  has several interesting properties that attract numerologists.  It is equal to the squares of the first 7 primes

and is a sum and difference of the first three 6th powers.

In addition, there are two ways to insert a plus sign "+" into the sequence 123456789 to make the 666, and exactly one way if the digits are reversed to be 987654321.
It is also a triangular number:  .  

A number having 666 digits has the ominous name of an "Apocalyptic Number."