Grand Unifying Guesses/Warehouse 13

Warehouse 13 is a massive Artifact of Doom collection thus it must have the following in inventory

 * Buffy's Scythe-Axe-Stake-Thing, The Hammer used to kick Glory's Ass, An Urn Of Osiris, Acathla, Illyra's casket and a few other things.
 * A few Denarii
 * Two rusted out, old fashioned revolvers.
 * The Ark of the Covenant
 * The Necronomicon
 * A Shining Trapezohedron
 * A Death Note
 * The One Ring
 * An alien costume
 * Horcruxes
 * Excalibur
 * The Witchblade
 * The artifact of Doom
 * A puzzle box
 * Umbra
 * A shard from the Jewel of Four Souls
 * The picture of Dorian Gray
 * Confirmed in the fourth episode of the second season.
 * Venjix's dormant code and a laptop computer
 * A fragment of the Allspark
 * The Pandorica
 * A chunk of pure smoking hot Evil
 * The original weighted Companion Cube.
 * A box of Turkish Delight
 * A Marker
 * The Necroteuch and the Malus Codicum.
 * An Anti-Music Missile
 * Soul Edge, and the sword forged to counter it, Soul Calibur.
 * The T-Virus and all of it's derivatives
 * Shinnok's Amulet
 * Vendredi's Cursed Antiques
 * A Risen Mitten
 * Old alien war machines
 * A Leprechaun's pot of gold.
 * A Delorean (possibly a steam engine train too), and the Flux Capacitor
 * A cursed VHS tape
 * A Halo Activation Index
 * Some pieces of a Reaper.
 * The Master Sword
 * At least one Keyblade
 * The Hakurei Yin-Yang Orbs
 * Several rolls of amateur student film.
 * And several more tapes.
 * And camera archives showcasing haunted people.
 * And footage of Operation Hammerdown.
 * A Ouija Board
 * The Kahili Ring
 * The Lance of Longinus
 * The Holy Grail
 * The Spear of Destiny
 * The One True Cross
 * Or at least a fragment of it.
 * The African Monolith from '2001: A Space Odyssey'
 * The Neverending Story, and the Auryn.
 * An Alethiometer.
 * A Sangreal.
 * A hunk of kryptonite.
 * and a Phantom Zone generator.
 * The Jumanji board game.
 * The Mask.
 * A 1st Edition Dungeons & Dragons manual.
 * the Gran Grimoire.
 * A piece of the Map of Inifinity
 * A certain other mask, only this time holding the power of a demon
 * A mask holding the power of a demon, you say?
 * The Hand of Glory.
 * A Starmap
 * A Prothean Beacon
 * The Shiftblade
 * A lance and sheild
 * A Polybius game
 * The Amulet of Yendor
 * A rotted hand and eye
 * The seven Crystal Stars and the eight Pure Hearts
 * A copy of the manuscript for William Shakespeare's Love's Labour's Won
 * The Book of the Dead, the Book of Amun-Ra, the Bracelet of Anubis, the Spear of Osiris and the Eye of Shangri-La
 * The Philosopher's Stone
 * The Witchblade
 * A copy of the Sburb beta
 * One or more of the Millenium Items
 * One or more Dragon Balls
 * The script for The King in Yellow (French original, with annotated English translation attached)
 * A Tibetan fife
 * A bobble-headed devil doll which produces fortunes
 * Marvin
 * A piece of Unobtanium.
 * The "Zoltar Speaks" wishing machine.
 * The Émigré Manuscript.
 * At least one lantern ring and power battery
 * A time lock portal
 * A super awesome axe known simply as The Seperator
 * The Sword of Omens
 * The BFG 9000
 * Murray
 * the really-really-bad-inator
 * A Timing Device
 * A golf club that causes people to take up beliefs similar to Ayn Rand's.
 * a boomerang
 * A piece of Eden
 * The Coins of Zakynthos.

Possible future artifacts

 * Dick Cheney's shotgun: makes the shooter dangerously innacurate
 * Weird Al's accordian
 * A syringe belonging to Keith Richards. An injection with it renders the subject immune to all poisons and ill side effects of drugs and narcotics.

Also see: SCP Foundation Grand Unifying Guesses, which Warehouse 13 may be controlled by.