Combat Colin, star of his own backup strip in the UK Action Force and Transformers comics, was known for his Combat Trousers (apparently presented to him by an alien), the pockets of which enabled him to produce any number of cumbersome weapons, up to and including a nuclear warhead.
A running gag in Peanuts is the incredible amount of space inside Snoopy's doghouse (although the reader only ever sees it from the outside). Reportedly, he can have huge parties in there. And he proudly displays his Van Gogh on one of the walls. There are at least two floors inside, as well — one strip depicts Snoopy listening to Linus and Charlie Brown negotiating a turn in a staircase while moving furniture.
At least one other comic series re-used the trick. Cubitus, star of his own Franco-Belgian comics by Dupa, also has a lavish palace inside his doghouse. In another story, he decides to move in his master's canopy bed. Friction ensues.
In The Mask, unlike the movie, while the wearer of the Mask can, if he/she/it decided to, pull an object from a pocket or inside their coat. Sometimes weapons can appear in the wearer's hand without them even realizing it. Sometimes they will actually kill people with a machine gun and then start wondering where it came from. Other times, it's explicitly shown that objects (weapons, usually) can in fact appear out of thin air in front of the person wearing the Mask. Even when objects are pulled out of pockets, most likely the object was not there before the wearer put their hands in there.
It's suggested the Mask uses the space between panels as hammerspace, which since the wearer is clearly unaware of being a comic book character is bound to be confusing.
In Warren Ellis's Planetary, Elijah Snow discovers that the Four invaded an alternate universe, slaughtered everyone living there, and turned it into an armory. A cruel way to invent Hammerspace.
Marvel Comics character Devil-Slayer lives this trope; he has a magic "Shadow Cloak" that allows him to pull weapons of virtually any sort (mostly swords, axes and other Hawkman-approved implements of destruction, but has included modern firearms and high-tech ray-guns). In the same universe, Corsair (of The Starjammers) uses "phasing discs" built into his gloves to pull blasters from a dimensional pocket and Rom, Greatest of the Spaceknights, summons his Translator, Energy Analyser, and Disruptor from "subspace"and sends them back again in the blink of an eye, when he needs them.
In Archie Comics, (particularly the older ones) whenever a character has just overcome something stressful or difficult, they will produce a handkerchief from nowhere and dab sweat from their forehead.
Done humorously in one issue of Marvel's Ultimate universe when a completely naked Hulk walks into a diner and asks for pancakes, then somehow produces a large handful of bills to prove that he can pay.
Wolverine's claws are many times longer than the backs of his hands from which they emerge. It's occasionally suggested that they're stored in his forearms, but since they come out with his wrists bent at any angle, it's much better sense that they're kept in hammerspace.