For instance, The Infinity Gems/Infinity Gauntlet from Marvel Comics, or maybe the Cosmic Cube, Thor's Hammer, The Eye of Aggamoto, and so forth... and of course Green Lantern's ring and such from DC comics... naturally, you could have the original owners turn up to claim their rightful property! In fact, I'd love to see Planescape versions of Thanos, Adam Warlock, etc...
Planescape, Dungeons & Dragons, their logos, Wizards of the Coast, and the Wizards of the Coast logo are ©2008, Wizards of the Coast, a subsidiary of Hasbro Inc. and used with permission.
Powers, what have I not stolen?
In my Slayers game I have six Noble Mozoku Chimera Templated out as one of the six Infinity Gems (Gaav's Children).
In my Ravenloft Game there is a Monkey's Paw...
In the Demiplane of Time I have a race of ChronoElf. One of which uses a two-dimensional sword (no thickness) so sharp in can cut portals of Time and Space in reality (copy of one of the X-Factor Character from one of the Annuals).
Lightsabers (pre-SWD20)
Nothing is sacred.