Cool - I'd been meaning to post a thread asking about Optimum Equipment Lists.
Some items I always get:
A large blank book and pens - maps and notes are always useful.
Chalk - marking where you've been.
Rope.
A week of rations (it's surprising how often people forget this).
A water skin.
Spare clothes and a blanket to sleep on/under. (Also often forgotten.)
Flint and steel, tindertwigs, or matches/lighter (if the setting has them).
A bandolier - for potions, scrolls, tool, and thrown weapons.
Wax for earplugs.
Mirror for looking under doors and around corners.
A lantern.
A pole composed of detachable 1 foot segments.
A prybar.
A backpack, and an extra empty sack.
A pouch of flour.
A bag of marbles.
A couple of daggers as utility knives, back-up weapons, etc.
One each of the splash damage items - acid, oil, alchemist's fire, thunderstone, tanglebag, etc. Especially the acid and the oil, which see a lot of non-combat use. (I've used acid to destroy runes on a magical trap, for instance).
A dog (or wolf), either as a familiar, animal companion, or just a well-trained pet.
Some kind of first aid kit.
I usually wind up needing a pack animal or extradimensional storage device pretty quickly.