Repeated shape for depth
How to use it
Duplicate the shape of an element, reduce the size slightly, place it behind that element, and make it a different colour (Darker, for example).
When to use it
This adds a sense of depth to a shape in a way that's not a drop-shadow or border. It's more visually interesting since, as of this writing, it's novel.
What to watch out for
This makes the shape look like a stack of two cards, rather than a single shape, so you might confuse your user.