I’m shocked to say this, but the cheap $20 knockoff “3D pens” from China actually work, at least as well as you’d expect such a thing to work.
These pens are basically little 3D printer heads in pen form that let you extrude PLA or ABS plastic in 3D space. It heats in about 30 seconds, extrudes with good consistency with controllable speed, runs on the same cheap 1.75mm filament as a printer, and the extruded plastic can actually stand up. It works best you have a fan going over the work area to cool the extruded stuff faster, and you need a bit of a blob for the base to keep it stuck, but you can actually draw into 3D space with it. At the base, getting adhesion is similar to 3D printing: masking tape (especially cleaned with alcohol) is a good surface.
I’m not sure how many 3D creations I’d make from scratch with this, but it seems absolutely fantastic for touching up or repairing 3D prints.
UPDATE: You can even make (very) light-duty brackets.
UPDATE 2: You can also use it to make fridge magnets of dubious artistic merit.