Gizmos.DrawCube  

function OnDrawGizmosSelected () {
// Draw a semitransparent blue cube at the transforms position
Gizmos.color = Color (1,0,0,.5);
Gizmos.DrawCube (transform.position, Vector3 (1,1,1));
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnDrawGizmosSelected() {
Gizmos.color = new Color(1, 0, 0, 0.5F);
Gizmos.DrawCube(transform.position, new Vector3(1, 1, 1));
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnDrawGizmosSelected():
Gizmos.color = Color(1, 0, 0, 0.5F)
Gizmos.DrawCube(transform.position, Vector3(1, 1, 1))