Not the most interesting thing I’ve ever made, but it shows how useful these printers can be. I had a small hole in my drywall, and didn’t have any patches available, so decided to design and print one using transparent PETG. 2 mm holes spaced 4mm center-to-center. Only 0.2 mm thick, so took very little time to print. One edge stuck too well to my TH3D build plate, but it got the job done! Attached to the wall by coating the back with spray adhesive, or you can put a thin layer of drywall mud/plaster, apply patch, then apply thin layer of mud/plaster on top. Since it's only 0.2mm tall it barely sticks up, so feathering the repair is easy.

Advice given to me by a professional - use a hammer or tool to sink the edges of the mesh flush or below the level of the drywall (before applying mud). It will be much better looking in the end!

11-16-2019: Added new square and rectangular patches, and patches to repair drywall around single and double gang switch boxes as well as round ceiling boxes.