91

91 = 7 x 13.

91 is the first non-trivial odd composite number. Every smaller composite is either even, a familiar square, ends in 5, has a digit sum that is a multiple of 3, or is obviously divisible by 11.

91 is the sum of the squares of the first six integers: 1² + 2² + 3² + 4² + 5² + 6² = 91. It is the 13th triangular number.

91 is the sum of the first 13 positive integers: 1 + 2 + 3 + . . . + 11 + 12 + 13 = 91.

91 has a unique representation as a sum of three squares: 91 = 1² + 3² + 9².

91 is 111 in base 9.

91 cents is the total amount of money one has, given one each of a penny, nickel, dime, quarter, and half dollar.
Source: Number Gossip