Sadly, THE iconic phat lewt container from all RPGs is sorely absent from ADOM. This is Jade, however, and we should think big. So let me present, without further ado, .. drumroll... the chest.

What does it look like?
There are many possibilities: 0 (just like a pool, the same coor as dungeon floor tile), ^ just like a a trap, or & like the forge. Personally, I vote for the first. It may change color to indicate if it's closed or open, in a manner similar to the pit trap.

How does it behave?
When you approach it, the game generates a text: A chest with a -/- lock is standing here. It is closed. (There may be variations: an old rusty chest, a small chest, battered, woden, whatever).
If it's opened, the text should say so, and tell if there are items inside, or if it is empty.

You can pick up the chest (should be heavy, starting from 1000stones, depending on material and what's inside), using the 'p'ick command, the game will ask you: Do you want to pick up the chest? y/n.

If you decline, you'll proceed to pick items from the floor (if there are any) and from the chest.
If you 'd'rop items, you're asked if you want to fill the chest, or drop them to the floor.

'o'pen command used on the tile with the chest opens/closes the continer. The chest may be locked, if so, lockpicking has to be applied. Kicking it open also works (chest traps anyone, maybe chance to damage items within? :]).

Chests have limited storage space, based on their size (say, it stores 2x the weight of the chest worth of items)

What good does it bring?
Common, we have chests. Chests are cool. That's iron logic for you right there. (-:
Items do not lie around anymore in disorder.
NPCs have places where they store their belongings. Gives options to thieves in cities.
More use for lockpicking.
If the code's in, we can have things like backpacks, pouches, sacks, etc. Though it may be pushing it a bit.
I think regular Chests are enough for most of our needs.

The possibility for artifacts:
Bag of Holding: no matter how much stuff you put inside, it weighs the same. But there's a limit to how much stuff will fit in.
Bag of Many Things: generates random stuff inside. Useful stuff, useless stuff, dangerous stuff. Would you dare to have a look?

EDIT: The idea was first proposed by hrothgar, btw. I simply expanded it a bit.
http://www.adom.de/forums/project.php?issueid=41