File:Arduino 1.0 IDE, Ubuntu 11.10.png

/*
  Blink
  Turns on an LED on for one second, then off for one second, repeatedly.
  
  This example code is in the public domain.
 */

void setup() {
  // initialize digital pin as an output.
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);   // set the LED on
  delay(1000);              // wait for a second
  digitalWrite(13, LOW);    // set the LED off
  delay(1000);              // wait for a second
}