Slinky cords are monofilament lines that are wound up like a coil. Useful for both speargun shooting lines and keeping equipment from falling off you, they are compact, out of the way, and when on a speargun, can speed reloading time. The inventor and commercial manufacturer of these cords is ACL Enterprises, and they and make the best ones. But they are a little spendy for the poor retro diver. You can make a cheap one from spare parts you probably have lying around.
You'll need some scrap monofilament, crimps, and a 1/2" dowel (about 8" long). Cut the mono to the length you want the line to be. Drill a small (3/32") hole in one end of the dowel (If you don't have a dowel around, the handle of a wooden spoon can work). Feed about 4" of the mono through the hole and then start wrapping it tightly around the dowel. When you have wrapped it to the desired length drill another hole and thread the mono through it.
Put the whole contraption in boiling water for 5 minutes or so. Remove from water and place in a bucket of ice water. Wait another 5 minutes and remove. Remove the ends and crimp them into a little "U". Done.