IMPROBABLE SHELF BRACKET 20cmx20cm
This shelf bracket is a true tensegrity structure, using PLA filament as the tension member, and the shelf itself working in compression against the wall to hold it up. Therefore the more you load it the more load it can carry, until the point of course where it can't. Don't do that!
It looks a bit improbable, and until it is actually on the wall you will doubt your sanity as it wobbles and wiggles while you carry it, but it stiffens up as soon as it's in place!
Mine has quietly held my cycling gear for well over a year, I've load tested it to 5kg because that's all I need it to carry, and while there was not a sign of stress, I suspect that at some point of loading the end will be sudden and spectacular!
The weakest point of course will be if it's loaded on the back edge only, causing the bracket to buckle. If that truly worries you (and if it does you probably shouldn't be building this!) you could screw one of my little angle brackets on the top edge for peace of mind.
The shelf pictured is 250mm wide (10") and the bracket projects 200mm (8")
Print in PLA, 25% infill, and if I were doing it again I'd use concentric top and bottom layers because they look better with the filament running in the direction of the load.
20cm x 20cm