This week's selection is a purely functional design: the Filament Clip by Thingiverse maker Elk.
There's not much to say about its structure, because the value of this design is in its function. It simply holds the end of a filament against the spool, as you can see in this image by Thingiverse maker rychuwit.
Why is this so important? This solves a truly evil problem faced by anyone using plastic filament-based 3D printing: tangles. Often makers lazily store their unused filament spools with loose ends. These ends sometimes slip under another strand, especially when the filament has a weak curl strength and unreels if not secured.
These tangles cause problems during prints because they'll eventually result in a knot and your filament feed will seize. This is particularly annoying if it happens deep into a multi-hour 3D print.
But that won't ever happen again, because you'll be using the Filament Clip!