MonoBehaviour.OnDrawGizmos()

Description

Implement OnDrawGizmos if you want to draw gizmos that can be pickable and are always drawn.

Allows you to quickly pick important objects in your Scene. When you select a child GameObject's gizmo, it selects the parent GameObject. this is because pickability is based on the callback’s host GameObject, not the transform used for drawing. To make a child GameObject’s gizmo directly selectable, implement OnDrawGizmos or OnDrawGizmosSelected on the child GameObject’s component.

OnDrawGizmos uses a mouse position that is relative to the Scene View.

Note: If Auto-hide gizmos is enabled in the Scene View preferences, then OnDrawGizmos is not called on components that are collapsed in the Inspector.

Use MonoBehaviour.OnDrawGizmosSelected to draw gizmos when the GameObject is selected.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void OnDrawGizmos()
    {
        // Draw a yellow sphere at the transform's position
        Gizmos.color = Color.yellow;
        Gizmos.DrawSphere(transform.position, 1);
    }
}

Additional resources: OnDrawGizmosSelected.