In Jack Vance's pulp fantasy Cugel's Saga, keeping track of your items is a big deal. Cugel the clever loses his items almost as soon as he gets them - stolen, smashed, misplaced, or most often, abandoned as Cugel flees from an angry mob. Cugel doesn't track encumbrance, but placement; because his items occupy a definite space in the world, everything in the world can effect them.
By contrast, items in DnD occupy a vague hammerspace somewhere around your character, where they can be teleported into your hands at any time. Traditional encumbrance systems work to weigh you down, but not to define where your items actually are. It's almost impossible for a DM to fuck with items in this hammer-space - you have to ask where the player's keeping them, they have to make something up, and you have to shrug and say "Well, they're gone now." It feels like cheating.
No more! I present Morgan Brackish Meadows' Anti-Hammerspace Item Tracker (I helped). This item-tracking sheet is designed to ground your items in the world without wasting a second of time at the table. Each character can carry 6 containers, and each container has 3 slots. The player defines what each one is - a sack, a utility belt, a backpack, whatever. Wearing Leather armour or better takes 1 container, Chain or better takes 2, Scale or better takes 3. The player just writes down or draws each item in a slot as they get it.
Stoner the Warrior
Cray the Thief
Some guidelines: A dagger is 1 slot, a short-sword is 2 slots, a longsword is 3 slots. 1 weeks food for one person is one slot. In my campaign you can get a +1 AC shield that takes up 1 slot, or a +2 AC shield that takes up 2 slots. Misc is for anything you're wearing, your animals, tiny objects, and anything else that wouldn't take up a slot. If you really want to add encumbrance, give them -5 feet of speed a turn per container. For mounts, use this same sheet and cross off containers based on the armour of their rider.
This is a harsh system that will see fully-clad warriors carry little more than food, weapons and gold. Personally, I love that - Conan never had much more than a loincloth and a sword. You can still own as many items as you like - it's just that you'll have to write them under "Left it at home", rather than hulking all your worldly possessions around on your back as you fight.
I have never seen anything but a game show the protagonists worried about their items weighing them down. Worrying about where your items are, though, is a staple of Pulp. Your arrows fall out, your money-pouch is stolen, your food gets wet, a dinosaur eats your back-pack and your bedroll is set on fire. That's a pulp encumbrance system. A few items that matter, constantly under threat.
I've been trying to capture that feel for a while now. One of my PC's was suspended by vine-tentacles over a carnivorous tree, and decided to throw lighted lamp-oil down into it's maw. Instead of teleporting it into his hand, I had him fumble with his backpack, holding flint and tinder in one hand and oil in the other, precious food-packs falling out into the maw as he tried to light the oil.
Encumbrance systems have always tried to tie your items into the world. They fail, not just because no-one can be bothered to do the math, but because they track weight instead of where the items are. This system solves that problem, and it feels like the only thing I could ever use. I'm going to try it in my next session, so I'll tell you how it goes.