ObjectPlacer

using UnityEngine;
using UnityEditor;
using System.Collections;

public class ObjectPlacer : EditorWindow {
    public GameObject prefab;

    [MenuItem ("Window/Object Placer")]
    static void Init () {
        ObjectPlacer window = (ObjectPlacer)EditorWindow.GetWindow (typeof (ObjectPlacer));
        window.Show();
    }

    void OnGUI() {
        prefab = EditorGUILayout.ObjectField ("Prefab", prefab, typeof(GameObject), true) as GameObject;
    }

    // Setup to be called by OnFocus and OnDestroy
    void OnSceneGUI(SceneView sceneView) {
        if (!prefab)
            return;

        if (Event.current.type == EventType.mouseDown) {
            RaycastHit hit;
            bool didHitSomething = Physics.Raycast(HandleUtility.GUIPointToWorldRay(Event.current.mousePosition), out hit);

            if (didHitSomething) {
                GameObject instance = Instantiate (prefab) as GameObject;
                instance.transform.position = hit.point;
                Undo.RegisterCreatedObjectUndo (instance, "Undo Instantiate " + prefab.name);
            }
        }
    }

    void OnFocus() {
        // Add or readd delegate
        SceneView.onSceneGUIDelegate -= OnSceneGUI;
        SceneView.onSceneGUIDelegate += OnSceneGUI;
    }

    void OnDestroy() {
        SceneView.onSceneGUIDelegate -= OnSceneGUI;
    }
}