[aequitas]

The option key works on many menubar items and also on menus.

For menus you can even press the button while it's open to see the items that change their behaviour with the option key. And it sometimes extends to keyboard shortcuts. For example, for logging out you can use shift-cmd-q and you will get a window with a choice, holding option along will log you out directly. In the menu this is shown by having '...' or not.

And on the topic of windows, you can cmd-click the titlebar of a window that is not the front most one and move it around whilst keeping it in the background.