GPS

compass

spare batteries

small towel/rag/bandanna

printouts of caches 

cache goodies for trade

pen and notebook

small first-aid kit

2-way radio or cell phone

camera

lighter/matches

permanent marker (for labeling caches)

ribbon to mark trails

calculator

pocket knife and/or multi-use tool

snacks and water

wet wipes (good for washing hands, sweaty faces, toilet paper)

work gloves (for reaching into places for the cache)

flashlight (even in the daylight. You never know what hole you'll have to climb or look into)

signal mirror (or an old CD)

whistle

gloves and plastic bag ("Cache in...Trash out")

rain poncho

local maps

watch

insect repellent

pack shovel (or multi-tool shovel/axe/saw)

binoculars

sweatshirt or jacket

FRS radios (better than yelling to your partner/s)