MonoBehaviour.OnDrawGizmos()

Description

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

This allows you to quickly pick important objects in your Scene.

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.