1)
5-in-1 reflector/diffuser. Has at least 5 uses, probably closer to 5000
. Most versatile lighting tool I own for $25 or less. Mine's 60cm diameter and overkill in size for little subjects, but still folded its small enough that I can tuck it in a waistband and only look moderately absurd. Great on its own for shaping ambient, also pairs well with flashes.
2) A
small led light panel, Aputure Amaran AL - M9. This thing is the size of a fat stack of credit cards but more useful when in the middle of the bush. A handy light for fungus or other small low-speed forest dwellers, it's also become my go-to as a focusing aid at night when using flashes. I do love my external flashes, but given its size and versatility this thing is almost always with me.
3)
UV flashlight. A new addition for me. My main purpose was to find caterpillars and other insects or spiders at night, but UV induced fluorescence is pretty interesting to photograph. Some caterpillars I've photographed are
here, but I've been aiming it at all sorts of other things (moss, lichen, mushrooms, algae, flowers, etc).
Can I squeeze in an honorable mention? My stack of
field guides and reference books. Pretty much the only books I actually purchase anymore. The latest additions cover Wildflowers, Fungus, Moths, Butterflies, and a dedicated text on Tent Caterpillars. I like to know what I'm photographing, both out of interest and to better be able to find and photograph them.