On my keychain, I carry keys. Mind you, it's a big carabiner clip full of keyrings which in turn are full of keys, but the main purpose is keys.
I used to carry more keys, but I started wearing out my belt loops. Now most of the keys (shhh, don't tell anyone) live in the glovebox of my car, all mounted to a gigantic split ring.
More important to me than keys (of which the only critically important one is usually a singular car key), are tools.
In my left pocket, I carry (or used to: I want another one desperately) my favorite screwdriver. It's a small forged flat blade screwdriver, made from steel that is both very hard and very -- shall we say -- tough. It turned all manner of screws, both phillips and flat and torx and other, and also made a handy deburring tool and I lost it a few months ago: If anyone has seen a yellow-and-black handled screwdriver, small, sold in ~1996 under the Vermont American "The Claw" brand in a set of 4 tools (three of which were useless), I will reward you kindly.
The left pocket also carries a very small stubby Phillips screwdriver, and pocket change, and has a lock-back Buck pocketknife clipped into it that varies between scary-sharp and near-useless depending on what I've been doing with it lately: I'm not nice to my knife, but I do try to take care of it.
The right pocket gets the phone, and only the phone. Despite Gorilla Glass and a TPU case, the pocket computer ("phone") gets its own place. (Yes, it was "only $199," but I won't be spending $599 off-contract to replace it and repairability has nose-dived since the OG Droid, which was simple to tear down and reassemble.)
The other right pocket (the one on my thigh) has my wallet: I used to carry my wallet in my rear/hip pocket, but it was killing my back from sitting on it. In the wallet, amongst a million discount and membership cards and whatever cash I might have, along with the Paypal/Simple/Local-credit-union debit cards, is a credit-card sized toolkit. It has my Courthouse Knife (never go anywhere without a knife / they never check the wallet), which I use for working in courthouses (it is always scary-sharp because it is seldom used and therefore tends to get touched up between uses), and some tweezers, a toothpick/reset-button pushing tool, a can/bottle opener that barely works (but does work well enough that I don't also carry a P38), and a very mediocre red LED flashlight which is a Godsend when faced with complete darkness (and absolutely useless the rest of the time).
The other left (thigh) pocket has nothing, unless I'm using hand tools, and then it tends to fill up with wrenches and pliars.
This entails that I wear cargo pants/shorts exclusively. I'm OK with not ever wearing jeans again. My butt-pockets never carry anything, and in fact I'd like to find clothing that doesn't have them at all.
And that's about it, except for the shirt pocket, which carries my hand-made smokes in a metal case, and a Bic lighter (Bic because they make the most consistently-reliable lighter, not because they make the most-efficient lighter). (And I don't wear shirts without pockets. I badly gimped my knee once because of a shirt with no pocket and spent a consecutive spring and summer stumbling around like a cripple because of that shirt: Never again.)