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.

Note that OnDrawGizmos will use a mouse position that is relative to the Scene View.

This function does not get called if the component is collapsed in the Inspector. Use OnDrawGizmosSelected to draw gizmos when the game object 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.