shortest and most effective code ever written

In my view without a doubt, it is this code snippet:

#include <stdio.h>
int main ()
{
    printf("Hello world!\n");
}

That's one line, and it's started a million coders on the path to programming. That's pretty effective.

Update: After many months, it's obvious that I need to be more direct. This code was effective without ever being executed. Not because it actually caused transistors to change state, but because it caused neurons to fire. In that regard, it has no equal.