Simple corner for holding credit cards, business cards, pictures or photos on any flat surface. Just add some self adhesive tape, foam tape or magnetic tape to the back. Prints without support.
Print two of them and you have a handy place to store a card (e.g. with your wifi router details). Print four to permanently mount a picture. You could probably use Blu Tack for a less permanent fix.
The overall size is 20x20mm and the card slot is 1.5mm wide, which should suit most credit cards (I measured ten, which ranged from 0.7mm to 1.26mm thick).