Gizmos.DrawWireCube  

function OnDrawGizmosSelected () {
// Draw a yellow cube at the transforms position
Gizmos.color = Color.yellow;
Gizmos.DrawWireCube (transform.position, Vector3 (1,1,1));
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnDrawGizmosSelected() {
Gizmos.color = Color.yellow;
Gizmos.DrawWireCube(transform.position, new Vector3(1, 1, 1));
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnDrawGizmosSelected():
Gizmos.color = Color.yellow
Gizmos.DrawWireCube(transform.position, Vector3(1, 1, 1))