How to do it...

To create a Gizmo to show the selected object in the Scene window, follow these steps:

1. Create a new Unity 3D project.
2. Create a 3D Cube by going to Create | 3D Object | Cube.
3. Create a C# script class called GizmoHighlightSelected and add an instance object as a component to the 3D Cube:

    using UnityEngine;
 
     public class GizmoHighlightSelected : MonoBehaviour {
         public float radius = 5.0f;
 
         void OnDrawGizmosSelected() {
             Gizmos.color = Color.red;
             Gizmos.DrawWireSphere(transform.position, radius);
 
             Gizmos.color = Color.yellow;
             Gizmos.DrawWireSphere(transform.position, radius - 0.1f);
 
             Gizmos.color = Color.green;
             Gizmos.DrawWireSphere(transform.position, radius - 0.2f);
         }
     }

4. Make lots of duplicates of the 3D Cube, distributing them randomly around the scene.
5. When you select one cube in the Hierarchy window...