Minimum code to Arduino USB Keyboard using v-usb (from http://rodolfo3.wordpress.com)

Arduino USB Keyboard

// for Atmega328P
#define USB_INTR_VECTOR INT0_vect

#include "UsbKeyboard.h"

// To use USB, the interruption Timer0 is disabled,
// breaking the default delay function from Arduino
// Replacing by a new one
void delayMs(unsigned int ms) {    // Safe delay helper function
  for (int i = 0; i < ms; i++) {
    delayMicroseconds(1000);
  }
}

void setup() {
  // Disable timer0 since it can mess with the USB timing. Note that
  // this means some functions such as delay() will no longer work.
  TIMSK0&=!(1<<TOIE0);
  // Clear interrupts while performing time-critical operations
  cli();
  // Force re-enumeration so the host will detect us
  usbDeviceDisconnect();
  delayMs(250);
  usbDeviceConnect();

  // Set interrupts again
  sei();

}

void loop() {
  UsbKeyboard.update();

}