C

Language Reference
Modified I/O sample from kattis help pages

#include <stdio.h>

int main(void) {
    char str[1000];
    while (fgets(str, 1000, stdin)) { //Or scanf
        printf("%s", &str);
    }
    return 0;
}

C++

Language Reference
Modified I/O sample from kattis help pages

#include <iostream>

int main(void) {
    long long data;
    while (std::cin >> data)
        std::cout << data << std::endl;
    return 0;
}

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNextLong()) {
            long data = sc.nextLong();
            System.out.println(data);
        }
    }
}

JavaScript (NodeJs)

const readline = require('readline');

const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

rl.on('line', (line) => {
    let data = parseInt(line);
    console.log(data);
});

Octave

Language reference
Solution to simple problem.

[name,count,read,err]=scanf("%s %d","C");
for j =  1:count(1,1)
  printf("Hipp hipp hurra, %s!\n", name);
endfor

Python

Local mirror of python documentation
I/O sample from kattis help pages

#! /usr/bin/python3
import sys

for line in sys.stdin:
    data = int(line)
    print(data)

Racket

If you use racket for programming contests, you should use #lang racket, not one of the teaching languages.
For language reference, run raco doc from the command line.
Sample solution to easy problem with numbers
Sample solution to a slightly more complicated problem. This one has formatted output and reading individual lines of input.
Code snippets showing input/output in racket

#lang racket
;; read stdin line by line, and output each line uppercased with a
;; line number
(define (upcase-stdin)
  (for ([line (in-lines (current-input-port))]
        [count (in-naturals)])
    (printf "~a: ~a~n" count (string-upcase line))))

;; read stdin into a list of numbers, return the list
(define (read-numbers)
  (for/list ([line (in-lines (current-input-port))])
    (string->number line)))


;; read stdin line by line, splitting into words.
(define (read-words)
  (for/list ([line (in-lines (current-input-port))])
    (string-split line)))