Invisible fastening hook. Works great for hanging small items in your workshop / garage. Printed both with PLA and ABS, PLA works just fine as you're not supposed to hang heavy items. ABS should provide extra strenght if needed.
I used it to store my pneumatic tools, bags...
Fasten the base with Ø4 coutersunk head wood screw, then just slide the hook on its base, you're done. You may adjust the little rib under it to fine tune the grab it provides.
Printed on Ultimaker 2+ with 0.4mm nozzle, 100% infill, 50mm/s. The rear of the hook should be the face on the build plate of the printer, with 10° supports it worked fine for me. The base should be printed with the countersunk face on the build plate also, with 10° supports.
Edit : improved design, added 1 mm to the hook base to reinforce it (was a little to thin).