Anvil of the Lortmil Mountains: Not an item I've used before but my it's one that any dwarven player would want. From Greyhawk Adventures, this anvil makes fine quality items which the book says is x100 value. That's a huge money maker!
Black Sails of the Schnai: Another Greyhawk Adventures item (it's my go to book), the Black Sails was always my favorite item in that book because of its Viking feel; a dead warrior is burned at sea and a scrap of this sail is kept so that his spirit can be called to fight again (as an einheriar). Who wouldn't want their character to have this sort of funeral?
Codex of the Infinite Planes: Here's an artifact that needs no introduction. I've never used it in a campaign that I recall, but I have used it plenty in my comics. In case you're inclined to go epic, there is no better source for Codex info than at Maldin's Greyhawk.
Dreamsinger: One of the five missing Blades of Corusk, this was the sword that gets the story going with its cryptic visions. I enjoyed Five Shall Be One and Howl From the North (except the end, no spoilers). Maybe someday I'll rework this for a new campaign.
Eye of Vecna: Another artifact that needs no introduction. I used to throw Vecna into my games so much in the 2e era that I had to swear him off for a decade or so. Perhaps the time is right? One of things I like about the Eye (and the Hand) is how a person has to remove his eye to use this item. In a game full of rules for everything, there is no game application for self-mutilation like this, but I'm sure it'd take intense willpower to rip your own eye out.
Fire Wand of the Suloise: Here's a neat wand I could never bring myself to dish out. It is said the wand may have been the device that caused the destruction of the Suel Empire (though wouldn't that make it the fire wand of the Baklunish?). Most likely its just a powerful reproduction of the "colorless fire" effect. One note, this wand can only be recharged on the Quasi-elemental Plane of Ash! Hey, I think Vecna lives there...
Greenswathe: Another of the five Blades of Corusk. I only bring this one up because it's an oddity among the adventure for being a scimitar. It's the only thing in the two adventures that sort of belies the viking feel. It is described as alien and arabesque, as well as being made of a golden metal with adamantine hardness. I guess much like the Vikings, the Thillonrians traveled to the Baklunish lands.
Heward's Handy Haversack: Is there a more ubiquitous magic item from Greyhawk than the Haversack? Later editions still use this item yet without his name attached, but we all know they are just like knock-off purses.
Iron Gauntlet of Urnst: Here's a gem of an item from GHA that I used on more than one occasion. Back in 1e/2e pummeling rules were slightly different than standard combat so the introduction of the Iron Gauntlet gave a great advantage (nevermind what a monk could do with it). Each hit you rolled on a small chart, gaining anything from a glancing blow (1d6 dmg) to a crushing blow (4d6 dmg + stun). Not too shabby.
Johydee's Mask: This is a minor relic from the old DMG that I never used but always intrigued me. Among its powers is changing into any humanlike creature, immunity to gaze attacks and mental reading. Hm, if any of my players are reading this, please disregard this entry.
Keoghtom's Ointment: Heward isn't the only quasi-deity who made functional items for all. Keoghtom's special healing balm has stood the test of time in D&D.
Lendor's Matrix: A strangely obscure artifact from the Scarlet Brotherood. Kevelli Mauk, founder of the Brotherhood used this hour-glass object to stop time and travel to the furthest border of the Suel Empire before the Rain of Colorless Fire took it. I don't think there is any official rules on this potent item, but if it can be found in a campaign it's surely held within the brotherhood's capital of Hesuel Ilshar.
Murlynd's Spoon: Ah yes, more utilitarian magic from Greyhawk's famous quasi-deities. For those who don't know, the spoon keeps creating barely edible (yet nutritious) gruel in empty bowls. With this, the haversack, the ointment and maybe a decanter of endless water, what more does a traveler need?
Nolzur's Marvelous Pigments: Nolzur is one of those lesser known personages of Greyhawk, but he does have an artistic theme to his magic, and that's a rarity in D&D. These paints sound hilariously fun and if you haven't read the entry in the 1e DMG I suggest you check it out. Basically whatever you paint on a 2D surface becomes real. Like a pit or door. It's like being Bugs Bunny!
Orbs of Dragonkind: Everyone knows about these artifacts partly due to the success of the Dragonlance Chronicles. However, when Roger E. Moore gave them an origin story in Dragon #230, it took the items back for Greyhawk in my opinion.
The Purple Stone: I will admit, it wasn't easy finding a "P" item, so this one will at least be interesting. It's not an item so much as a location in WG5 Mordenkainen's Fantastic Adventure (and Dungeon #112 Maure Castle). Not to give too much away, but if the stone can be found, it will impart visions or powers to worthy PCs. Let's hope that is enticing enough to get my game group back in Maure Castle.
Quiver of Ehlonna: Not to be outdone by Heward, the goddess Ehlonna decided archers don't have enough arrows so she created an item that makes counting ammo almost superfluous.
Razor Claws of the Chakyik: I love all the items in GHA because their origins are from all corners of the map. This item in particular turns your PC into freaking Wolverine! As expected, this is a pair of gauntlets with 4 curved daggers attached. The damage isn't 4d4 however, it's a paltry 1d6+2. Who cares? Think of the coolness factor.
Sureguard and Swiftdoom: Without a doubt, Kelanen the hero-god of swordsmen has the two best sentient swords in the game. I especially loved using these two swords in my old Greyhawk comics.
Theorparts: These are the three numinous objects that when assembled, can free Tharizdun in Gygax's novels. They were apparently never meant to be translated over to the game setting so anyone wanting to follow this dangerous path must consult this old source, which has the best info on the Theorparts.
Unopenable Doors: Another alphabetical cheat from Maure Castle. The doors are magical and they are objects, but nothing a player could possess. I always found it funny how the beginning of WG5 was at times harder than later portions. Indeed, if you can't open the front door to the dungeon, it's over before it begins!
Viper of Hades: Here's a deific magic weapon. The whip of Pyremius, god of murder and fire was featured in Dragon #89 is a nasty poisoned weapon which ignores dwarf, gnome and Halfling saving throw bonuses. Ouch!
Wand of Orcus: There are few D&D players who haven't heard of the demon-prince Orcus. His skull wand is one of those uber-artifacts in AD&D that just ignores the rules. If it touches mortal flesh you are dead. Period. Oh yeah, it may have other powers as well according to the Monster Manual, but does Orcus really need it?
Xenous (The Face of): Yeah, I'm really scraping the bottom of the barrel here. The Face of Xenous is a weird artifact found in Treasures of Greyhawk. The evil wizard Xenous Zenpor made this special wish to Nerull and not to ruin the adventure for anyone but in return she got this magic box. That's about all I'll say...
Yatil Wand of Zooming: Speaking of weird, this wand from GHA is quite different as well. It was originally devised to speed up travel from mountain to mountain. Yes, nothing bad could happen here. The wand can hurl a person, willing or not, at high speed. How they land is their own problem.
Zagyg's Spell Component Case: To finish out the list is another fittingly useful item made by the great Zagyg himself. The case as you would expect is an extradimensional device that conjures up whatever spell component you need when you reach in. If only they made dice bags like that.
Update 05/27/2021: Had to remove dead link to the Theorpart info. Sadly Tharizdun's secrets may just stay that way.