A metal tube with a grate near the bottom. Charcoal goes in the top, two sheets of newspaper go underneath, and the chimney effect lights the whole load evenly in around fifteen minutes.
It is on this list because it removes the two worst parts of cooking on charcoal. It gets you to usable heat faster and more predictably than lighting a heap in the base, and it means you never need lighter fluid, which is another flammable liquid to carry and a reliable way to make food taste of paraffin.
It also lets you add a second load part way through a cook without putting cold coals onto a dying fire, which is the usual reason a barbecue runs out of heat before the food is done.
The same warning applies as to the barbecue itself, and applies more strongly here because a chimney full of lit coals looks like a thing you could carry somewhere sheltered: carbon monoxide.