Unity3D script

LookAtPoint.cs

using UnityEngine;

[ExecuteInEditMode]
public class LookAtPoint : MonoBehaviour {
	
	public Vector3 lookAtPoint = Vector3.zero;
	
	// Update is called once per frame
	void Update () {
		transform.LookAt (lookAtPoint);
	}
}

LookAtPointEditor.cs

using UnityEngine;
using UnityEditor;

[CustomEditor (typeof(LookAtPoint))]
public class LookAtPointEditor : Editor {
	
	// Update is called once per frame
	public override void OnInspectorGUI () {
		LookAtPoint _target = (LookAtPoint)target;
		_target.lookAtPoint = EditorGUILayout.Vector3Field ("Look At Point:", _target.lookAtPoint);
		if (GUI.changed) {
			EditorUtility.SetDirty(_target);
		}
	}
	
	void OnSceneGUI () {
		LookAtPoint _target = (LookAtPoint)target;
		_target.lookAtPoint = Handles.PositionHandle(_target.lookAtPoint, Quaternion.identity);
		if (GUI.changed) {
			EditorUtility.SetDirty(_target);
		}
	}
}