Glowstick

It is the light source that works where a torch will not. Underwater it keeps burning, which by itself justifies a stack on any ocean trip or flooded cave dive. It cannot be attached to anything, so it lights wherever it lands and dies after a few minutes.

Use the attack key to throw one. The dedicated throw key does something else entirely, dumping the whole stack a couple of tiles away, which is a mistake everyone makes exactly once. Thrown at a shallow angle a glowstick will squeeze through a single block of wall the way a boomerang does, so you can scout the far side of a barrier before you break into it.

Drop one at your feet while mining straight down and it falls with you, so there is always light on the tile you are about to break. During the Torch God event, being able to light a room from a distance instead of placing torches by hand is worth more than it sounds.

Between jellyfish, pots and chests you tend to collect them without trying, and the Merchant stocks them cheaply at night if you ever run out.