SCP items labeled "ectoentropic" will likely implement this trope in some fashion since it means the object generates matter and/or energy in an anomalous manner.
SCP-049 ("Plague Doctor"). SCP-049 can produce a bag containing scalpels, needle, and thread from its body and return it to its body when it's finished with it. The bag cannot be detected in any way while inside SCP-049's body.
Destroy the Godmodder: Used frequently for everything from inventories to hiding, even used to trap people for short attacks sometimes.
Both Ruby and Tom in Ruby Quest have Inventories in which they can store up to 8 items.
Destroy the Godmodder: Everything, just everything. People pull weapons seemingly out of nowhere, and in DTG2, people began abusing this to utilize more weapons than they should have.
In the Tales of MU, the nymph Amarath has a habit of putting unneeded items Away.
In the New York Magician series, Michel is able to pull a magical flintlock pistol out of the air by raising his arm like he's hailing a cab.
The Legend of Neil (a Legend of Zelda parody) inverts this then lampshades it. For the first few episodes Neil awkwardly carries everything (including bomb, bow, sword, shield, and the raft) until a secret Moblin teaches him how to use hammerspace.
Played Straight in Metafictionized Phlebotinum Poisoning, where Arthur explains that the angels get 'P.L.O.T. hole' lockers, which are just regular storage units, but with mini-portals attached. Also explains how he drew his sword out of nowhere. Tagino gets one in chapter seven, too.
Lampshaded in Four Swords Misadventures episode 7 when Green reacts with confusion when a drunken Red manages to use a hookshot on Dark Link after being inadvertantly woken up by him (he earlier passed out), with Blue asking Green "how should [Blue] know [where Red got the Hookshot]?" after Green asked him.
In the Whateley Universe, there's a character codenamed Mobius who makes utility belts with pouches that are about ten times bigger in each direction than they are on the outside. Phase has one that looks like it couldn't hold a postage stamp. He carries everything up to a touch Taser and throwing knives in it.
In Dr. Horrible's Sing-Along Blog, after the eponymous character spends time walking around monologuing, he suddenly spins around and dramatically pulls out from under his lab coat a giant Death Ray that was quite clearly not actually under there before that point due to the way the coat hangs. note Confirmed in the commentary, where they note that they filmed the scene in two parts, "pre-gun" and "post-gun".The hammer space is his penis.
In this Conroy Cat short where Conroy pulls out several items throughout the cartoon that will help him from stop falling.
Invoked by Daniel in "On the Couch", who hides large weapons in convenient places in Curly's house.
Violet Capagio, a super hero in Skyway Mechanix, has pulling weapons from hammerspace as her main power.
In Worm , the villain Circus has a grabbag of minor powers including her own hammerspace, which she uses to store an actual sledgehammer that she uses in combat.
When transformed, the characters in The Cartoon Man are able to produce cartoon objects from thin air. Simon pulls out an actual hammer on multiple occasions.
A story in the comments of Friendship is Dragons takes this into full Heart Is an Awesome Power territory. The narrator was once in a toon-esque game where one of the things you could put points in was "Hammerspace Capacity". The narrator put all her points into that, and by a few sessions in had a Hammerspace capacity in the neighborhood of half a cubic light-year. Then she revealed why she'd done this. The party was sent to retrieve a mysterious artifact from a heavily guarded dungeon. The narrator simply stuffed the entire dungeon into her hammerspace, then pulled the artifact back out. Moreover, the rules also specified that anything you pulled out of your Hammerspace was yours to wield as you saw fit. Including the Big Bad's dragon minions. Needless to say, the Hammerspace rules were majorly tweaked after that campaign.