1 is an odd number.
1 is a palindrome.
1 is a Harshad number, which means it is divisible by the sum of its digits (1 = 1, and 1 is divisible by 1).
In binary, 1 is represented as 1.
In hex, 1 is represented as 1.
1 is an Armstrong (or Narcissistic) number, which means it is the sum of its own digits each raised to the power of the number of digits.
1 is the factorial number of 1 (1! = 1).
1 is the 1st Fibonacci number.
1 is the only number that is neither prime nor composite.