Er, oh dear. Yes. Yes indeed.
The Hand and Eye of Vecna and the Sword of Kas, of course. Nobody wanted to so much as touch the Hand/Eye, but the Sword of Kas (being intelligent and telepathic and with a monstrous ego score) managed to tun the characters against each other in very short order. Nasty, messy business.
I also had a "Lost Sword of A Hundred Hearts" that was a cursed berserking sword that allegedly turned into a vorpal weapon once you had killed 100 people. None of the wielders ever survived long enough to fond out (this one guy's pyromancer has it now, for some odd reason).
I had the Throne of the Gods in one adventure, many many years ago (hmmm, same one that they got the Lost Sword of a Hundred Hearts, actually). Terrible, terrible decision. Once they figured out how to operate its wish powers, the party were hopping on and off it like some game of musical chairs, while I was dispensing curses and counter-effects with equal rapidity. The year-long storyline was derailed, in flames and coughing its last within moments, a steaming heap before we realised what we were doing. There was an uncomfortable silence. Then one player says "Look, I'll just get back on and wish that we had never found the damn thing, ok?" Fate was more than willing to comply with that and they left it well alone.
A high-level group went back to the Throne in the final phases of the campaign, about 15 real years later. They used it once, once only, and promptly cleared off. Thankyouthankyouthankyou.
It wasn't an artifact back then, but had numerous mishaps with decks of many things. As a rule, there is one in every campaign I run hidden away somewhere. I never learn where this one is concerned, I guess.
Ummm, lemme see... Oh yeah, the Sevenfold Torc (which was the Rod of Seven Parts as a big necklace, really). We were all much older and wiser and warier by the time this one cropped up, so it was used sparingly and with great paranoia. And once the Queen of Chaos put in a brief appearance, not at all ever again.
A long-term party in my games called Tsunami tried three times to tackle White Plume Mountain and nab Black Razor, Whelm and Wave. It defeated them three times over, to my eternal glee. Bloody lunatics. My own character did get Black Razor a long time before that in another game. The flippin DM made me give it back!!! And he wasn't subtle about it either (I rant about this in Quasqueton's thread on White Plume Mountain, so I'll spare you the repetition here...)
In my first few months of DMing I had this thing for magical artifact swords, one for each alignment (so I had three to begin with and then all of a sudden there were six more with AD&D, heh heh). Naturally, my buddy Colin's character got the Lawful one and my own character got the Chaotic one in a game I also just happened to be DMing at the time (Isle of Dread, in the kraken's cave). He was with this "NPC party" at the time - you know how it goes

. The Lawful sword could shoot 20d6 fireballs and summon efreets and that kind of thing (it was a Fire sword, you know). The Chaotic one could shoot spheres of annihilation. No, really, it could. Not surprisingly, Colin organised the party to remedy this unbalanced situation and they all ganged up on my character and took his Chaotic Void sword and threw him into the sea. Then Colin gave the Fire sword to another player and gave the Chaotic sword to one of his other characters. So of course I made them lose the Fire sword in the volcano on the Isle of Dread and then made it erupt, forcing them to run away. Anway, like anybody cares. I ramble...
I had this other homebrew artifact thing that I cooked up for the last few adventures of my campaign, which finally featured deities manfesting on the Prime. Called the "Eye And Lens Of Tuvaanis", it is basically a big eyepiece that grafts itself to your head and replaces your eye with a gem-like orb. This allows you to see and target the divine chakras that gods use to maintain their divine state when manifesting in the Prime. Once targetted, you can then shoot laser beams out of your eye to destroy those chakras, degrading the gods power incrementally and allowing you to shmam them. You know the campaign is going to end when the DM gives you one of those. And now it won't come off

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Oh god, and then there was that flying mountain thing. For ages and ages it belonged to an uberNPC gold dragon, who laired there in a magical palace and used it to fly about the upper atmosphere, host PCs and generally be kewl. All the dragons died towards the end of the campaign and so of course the PCs got it. They were remarkably restrained, using it mainly for travelling to other planes and only really raining destruction down on the earth below once or twice.
Hey, wow they're really starting to come back to me now...
I had these three... swords... (yes, again!), one that was used to create the spirit world, one to create the material world and one to master the void between them. This last was never understood and was really only used to kill people, but the others featured and were used in a number of stories.
I had this big crystal mcguffin that was variously shattered, being reassembled or intact. It could do anything. The gods would do Really Awful Things to people who used it, but a couple of characters went ahead and used it anyway, one those occassions when a shard passed through the game. A plague got healed. People got raised en masse. The barriers of spacetime and continuity were forever breached. That kind of thing.
The Veil of Mugaal - a little, er, veil that made you invisible to everything and everyone, gods included. My girlfriend has that now.
The Black Staff of Charris/Manshoon/Billy - a killer staff of the magi that never seemed to run out of charges...
I could go on... Best not, eh?