Selection.activeTransform  

static var activeTransform : Transform

Description

Returns the active transform. (The one shown in the inspector)

This will never return prefabs or non-modifyable objects.

// C# Example
// Duplicates the selected object and places it at the origin

using UnityEngine;
using UnityEditor;

public class SelectionActiveTransform : ScriptableObject {
    [MenuItem ("Example/Duplicate at Origin _d")]
    static void DuplicateSelected() {
        Instantiate(Selection.activeTransform, Vector3.zero, Quaternion.identity);
    }

    //The menu item will be disabled if nothing, is selected.
    [MenuItem ("Example/Duplicate at Origin _d", true)]
    static bool ValidateSelection () {
        return Selection.activeTransform != null;
    }
}