Stroke as light bottom edge
How to use it
Add a stroke along the bottom of an area, and make it a lighter colour than the background colour of the area.
When to use it
This is quite rare, but I've seen it in some cases where the body of the design, like a website, is a bright colour, and seems to be casting light on the navigation area above it. This creates a lighter bottom edge.
What to watch out for
An object having a lighter bottom edge is not normal - we're used to objects having darker bottom edges since light usually comes from above. If you use this technique, try to make it obvious why there is a lighter bottom edge.