999

999 = 3³ x 37.

999 is the smallest possible sum of three-digit primes that together include all nine digits: 149 + 263 + 587 = 999.

The odd powers of 999 end with 999, and its even powers end with 001: 999² = 998001; 999³ = 997002999.

999 is 4343 in base 6, 2625 in base 7, and 1747 in base 8.

999 is the Dewey Decimal Classification system call number for library books on the general history of extraterrestrial worlds.

Source: Numeropedia