PrimitiveType.Cube  

// Creates a cube primitive
function Start () {
var cube : GameObject = GameObject.CreatePrimitive(PrimitiveType.Cube);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Start() {
GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Start():
cube as GameObject = GameObject.CreatePrimitive(PrimitiveType.Cube)