Easily one of the most
interesting numbers I can think of.
This is largely due to the fact that 153 is the
sum or
product of other
interesting numbers. Also,
imho, 153 is at least as interesting as
23, which is
widely known to be the
smallest interesting number.
The Basics:
The Interesting Stuff:

153 is the smallest number  other than 1  which can be expressed as the sum of cubes of its digits:
153 = 1^{3} + 5^{3} + 3^{3}.
See also: Narcissistic number.

It is equal to the sum of the first 5 factorials:
153 = 1! + 2! + 3! + 4! + 5!

The sum of digits of 153 is a perfect square:
1 + 5 + 3 = 9 = 3^{2}

On adding the number 153 to its reverse, 504 is obtained, which when squared is the smallest square which can be expressed as the product of two different numbers which are reverse of one another:
153 + 351 = 504
504^{2} = 288 x 882 (= 254016)

It is the 17th triangular number: it is the sum of all integers from 1 to 17. Since reverse of 153, i.e. 351 is also a triangular number, 153 can be termed as a reversible triangular number.

Its prime factorization is 153 = 3^{2} x 17.
3^{2} = 9 = 1 + 5 + 3, meaning that 153 is divisible by the sum of its digits. This qualifies it as one of the illustrious Harshad numbers (a.k.a. Niven number).

It can be expressed as the product of two numbers formed from its own digits:
153 = 3 * 51

153 is the 9^{th} hexagonal number. Hexagonal numbers are given by the formula H(n) = n(2n  1).

If all 6 combinations of number 1, 5, 3 are taken to form a three digit number then the following is true:
153 + 315 + 531 = 351 + 135 + 513
1^{0} + 5^{1} + 3^{2} = 1 * 5 * 3
But the weirdest, wonderfullest, wackiest factaboutanumber for today:

Work out its reciprocal
1/153 = 0.006535947712418300653594...
Multiplying all the significant figures (the repeated nonzero numbers after the point) by 17, and multiples of 17 the pattern generated is:
65359477124183 x 17 = 1111111111111111
65359477124183 x 34 = 2222222222222222
65359477124183 x 51 = 3333333333333333
65359477124183 x 68 = 4444444444444444
65359477124183 x 85 = 5555555555555555
65359477124183 x 102 = 6666666666666666
65359477124183 x 119 = 7777777777777777
65359477124183 x 136 = 8888888888888888
65359477124183 x 153 = 9999999999999999
Fairly Interesting Biblical Stuff^{©}:
wikipedia.org
worldofnumbers.com/em41.htm
shyamsundergupta.com/c153.htm