Rebuildable tank atomizer. Instead of paying through the ear (and waiting a month) for coils you screw in, you get 20 cm of an appropriate wire, wrap it around a nail/screwdriver/coil jig/whatever works for you, stuff some cotton or rayon through it, and 10 minutes later you have a 10 cent (or pence) coil that vapes beautifully (at least after some practice wicking. The first few tries may fail to vape, vape anemically or leak over everything.)
The
rebuildables forum is a good place to learn more and to get suggestions for RTAs that might suit you.