18,446,744,073,709,551,615

Number
$18,446,744,073,709,551,615$ is:


 * $3 \times 5 \times 17 \times 257 \times 641 \times 65 \, 537 \times 6 \, 700 \, 417$


 * $2^{64} - 1$


 * The number of moves it would take the monks to move all the disks onto a different peg in the Tower of Hanoi.

Also see

 * Prime Factors of $2^{64} - 1$