A program that calculates every Harshad number up to five hundred million (500,000,000 or 5×10^8).
According to Wikipedia, a harshad number is a number in a given base that is divisible by the sum of its digits when written in that base.
Basically, if I take a number (e.g. 954) and divide it by the sum of the digits (9+5+4), the answer should be whole: 954 ÷ (9+5+4) = 53, which is whole.
Because I can. :)