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)