A cheap metal disc that sits between the flame and the pan. It costs very little, weighs almost nothing, and fixes the single most common failure of cooking on a camping stove.
Most portable burners are designed to boil water fast, and their low setting is still fierce and concentrated in the middle of the pan base. A great deal of the food here wants twenty minutes at a bare simmer, and without help what you get is a scorched ring in the centre and a burnt bottom on anything with tomato, milk powder or sugar in it.
The diffuser spreads that heat across the whole base and takes the edge off it. Dal, porridge, rice, anything thickened with flour or semolina: all of them stop catching.
It is the least glamorous thing on this page and probably the highest ratio of improvement to money. See cooking on one burner.