Unity SceneAutoLoader Script from http://wiki.unity3d.com/index.php/SceneAutoLoader

SceneAutoLoader.cs

using UnityEngine;
using UnityEditor;

/// <summary>
/// Scene auto loader.
/// </summary>
/// <description>
/// This class adds a File > Scene Autoload menu containing options to select
/// a "master scene" enable it to be auto-loaded when the user presses play
/// in the editor. When enabled, the selected scene will be loaded on play,
/// then the original scene will be reloaded on stop.
///
/// Based on an idea on this thread:
/// http://forum.unity3d.com/threads/157502-Executing-first-scene-in-build-settings-when-pressing-play-button-in-editor
/// </description>
[InitializeOnLoad]
static class SceneAutoLoader
{
	// Static constructor binds a playmode-changed callback.
	// [InitializeOnLoad] above makes sure this gets executed.
	static SceneAutoLoader()
	{
		EditorApplication.playmodeStateChanged += OnPlayModeChanged;
	}
	
	// Menu items to select the "master" scene and control whether or not to load it.
	[MenuItem("File/Scene Autoload/Select Master Scene...")]
	private static void SelectMasterScene()
	{
		string masterScene = EditorUtility.OpenFilePanel("Select Master Scene", Application.dataPath, "unity");
		masterScene = masterScene.Replace(Application.dataPath, "Assets/"); //project relative instead of absolute path 
		if (!string.IsNullOrEmpty(masterScene))
		{
			MasterScene = masterScene;
			LoadMasterOnPlay = true;
		}
	}
	
	[MenuItem("File/Scene Autoload/Load Master On Play", true)]
	private static bool ShowLoadMasterOnPlay()
	{
		return !LoadMasterOnPlay;
	}
	[MenuItem("File/Scene Autoload/Load Master On Play")]
	private static void EnableLoadMasterOnPlay()
	{
		LoadMasterOnPlay = true;
	}
	
	[MenuItem("File/Scene Autoload/Don't Load Master On Play", true)]
	private static bool ShowDontLoadMasterOnPlay()
	{
		return LoadMasterOnPlay;
	}
	[MenuItem("File/Scene Autoload/Don't Load Master On Play")]
	private static void DisableLoadMasterOnPlay()
	{
		LoadMasterOnPlay = false;
	}
	
	// Play mode change callback handles the scene load/reload.
	private static void OnPlayModeChanged()
	{
		if (!LoadMasterOnPlay)
		{
			return;
		}
		
		if (!EditorApplication.isPlaying && EditorApplication.isPlayingOrWillChangePlaymode)
		{
			// User pressed play -- autoload master scene.
			PreviousScene = EditorApplication.currentScene;
			if (EditorApplication.SaveCurrentSceneIfUserWantsTo())
			{
				if (!EditorApplication.OpenScene(MasterScene))
				{
					Debug.LogError(string.Format("error: scene not found: {0}", MasterScene));
					EditorApplication.isPlaying = false;
				}
			}
			else
			{
				// User cancelled the save operation -- cancel play as well.
				EditorApplication.isPlaying = false;
			}
		}
		if (EditorApplication.isPlaying && !EditorApplication.isPlayingOrWillChangePlaymode)
		{
			// User pressed stop -- reload previous scene.
			if (!EditorApplication.OpenScene(PreviousScene))
			{
				Debug.LogError(string.Format("error: scene not found: {0}", PreviousScene));
			}
		}
	}
	
	// Properties are remembered as editor preferences.
	private const string cEditorPrefLoadMasterOnPlay = "SceneAutoLoader.LoadMasterOnPlay";
	private const string cEditorPrefMasterScene = "SceneAutoLoader.MasterScene";
	private const string cEditorPrefPreviousScene = "SceneAutoLoader.PreviousScene";
	
	private static bool LoadMasterOnPlay
	{
		get { return EditorPrefs.GetBool(cEditorPrefLoadMasterOnPlay, false); }
		set { EditorPrefs.SetBool(cEditorPrefLoadMasterOnPlay, value); }
	}
	
	private static string MasterScene
	{
		get { return EditorPrefs.GetString(cEditorPrefMasterScene, "Master.unity"); }
		set { EditorPrefs.SetString(cEditorPrefMasterScene, value); }
	}
	
	private static string PreviousScene
	{
		get { return EditorPrefs.GetString(cEditorPrefPreviousScene, EditorApplication.currentScene); }
		set { EditorPrefs.SetString(cEditorPrefPreviousScene, value); }
	}
}