Knowing how to write hello-world program in multiple languages is crucial to be called as hello-world programmer, so here is my collection.

C

#include <stdio.h>
int main() {
    puts("Hello world.");
    return 0;
}

Java

public class main {
    public static void main(String[] args) {
        System.out.println("Hello world.");
    }
}

Haskell

main =
  putStrLn "Hello world."

CoffeeScript

console.log "Hello world."