With scripting you can add menu items that have shortcut keys assigned to them. Read the MenuItem description for more information.

A great way to help debug your game is to use Gizmos to visually represent empty game objects in your scene.

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 When working with multiple duplicate game objects you may want use tags to uniqely identify a specific object.  You can then use GameObject.FindGameObjectsWithTag to select that specific object in your scripts.

If you need to output information to the unity console check out the Debug class.

If you need to set the screen resolution of your game via scripting check out Screen.SetResolution.

Did you know that you can manipulate a game objects collider in the scene view by holding down the 'Shift' key. Holding shift will display the colliders control dots. Just drag the dots to adjust the collider!

Hold the 'V' key while the translate tool is active and you can drag an object by it's vertices and snap that vertex to another objects vertex!

The inspector window contains a small lock icon in the top right of the window. If you select a game object and click the lock the inspector will be locked to that object and will not change when you select other objects. Click again to disable the lock. This allows you to have multiple instector windows open with locks to different objects.

To allow the user to undo actions that are performed by a custom editor script you can use Undo.RegisterUndo. 

foreach (var item in Selection.transforms)
{
    Undo.RegisterUndo(item, "Move " + item.name);
    var pos = item.position;
    pos = new Vector3(activeObject.transform.position.x, pos.y, pos.z);
    item.transform.position = pos;
}

If you want to take advantage of functionality that is provided my menu commands you can use EditorApplication.ExecuteMenuItem in your editor scripts to execute them.