Patterned solid drop shadow
Credit to Martin Pitt for suggesting this.
How to use it
Duplicate an element and put it behind that element, then offset it and overlay it with a dark repeating pattern, creating a shadow effect.
When to use it
If you want to add some branding to a bold drop shadow, and also create a retro look, this can be a good way to do it.
What to watch out for
This is quite bold, so it needs to make sense with your brand.
Simple patterns and dark colours will work best. If it's too bright or too flashy, you might find that the shadow distracts the user from the element casting it, which doesn't help them understand where they should be focusing.