Do you have any recurring NPCs that show up between campaigns, even in different worlds?

Technically yes, I make reference to shit I've made in other universes, but its obscure information and hardly relevant to the actual plot.

My first character that gained godly magic powers and in a fit of drunken power decided to smite a whole country with lightning the DM was so amused of the whole mess that decided to make him a recurring character

I've met fucking Crazy Hassan in Exalted. He sold us camels while we were in the south. He may or may not have been a sidereal. Or just an Exalted of 'Selling Camels'...somehow.

Does that count?

One of my players' characters ascended to godhood, and now she shows up as a regular greater deity whenever he DMs.

When I GM'd my group's second game, set in the same world as the first, I had my PC's daughter show up at one point to help the party

Shrivan'Nam, the Master of Hidden Ways. He's something between bbeg and trickster god, and by now my players know never to trust him unless they're really desperate.

My friend played a Druid in our first ever D&D campaign, and lost his life to an assassin during a rescue mission for our party's elf ranger, who had been captured by Drow. He wanted to try Warmage next, so we honored the Druid's final wish by burying him in a field and planting a single tree on his grave.

When I later DM'd a game, the party were lost in a forest that constantly moved itself and the party around randomly. They encountered the oldest tree in the forest, which was sentient, and had to negotiate with him to allow them to get where they needed. The Druid's (former) player was pleased when he asked the wise old tree's name. I plan on having him make another appearance at some point in our current campaign as well.

I'm planning to do something similar, I did some kinda of demo with my friends to show them D&D, we ended the session with the tower they were in collapsing and them dying, I'm trying to think of a way to subtle reference them in a real campaign later apart from just seeing some skeletons in rubble

Harry Jewdini. In games that don't specifically have a carrying capacity he shows up and steals the things my retard players try to loot (doors, hundreds of rifles, floor panels) that they absolutely shouldn't be able to carry.

Started in a Star Wars campaign were a guy was playing a Jawa force user and claimed he could take all the doors, plumbing, weapons, electronics, and bodies from the base they assaulted. Rather than deal with his protesting that he could carry whatever he wanted because there weren't any limits I just had the Jewdini sneakily steal all of what he wanted and bring it back to the dimension he came from.

He shows up now and again when anyone tries to do something equally dumb.

one particular dude in armor carved from solid rock

Amazing, you gave me just the fix I needed to not implement dumb carry capacity rules without the players taking advantage of it, thanks

Oh! You could bring them back as undead enemies!

Delores. A chain smoking, cat sweater wearing, middle aged New Jersey Jew in leopard print spandex pants. She will be found running at least one store front in every campaign regardless of system or genre usually with a gravelly "whaddya want hunny".

That's genuinely awesome

My group tends to rotate GMs, and we all tend to do some references to previous things we've done. Often by having NPCs who resemble PCs of prior games to make a silly call-back or two. I don't know if we have anything too recurring beyond the little in-joke references to our other games, though.

I have one NPC who appears as a "Bonus Boss". Like I was running a short campaign and the players found all the hidden " audio files" so they got to fight him.
He was the reoccurring badguy in out first game we ran.

All of my taverns have a cloaked and hooded man smoking a pipe in a dark corner. Every tavern. Same guy, even in multiple taverns at the same time, across campaigns and settings (he showed up in D&D forgotten realms, Shadowrun, Star Wars, everywhere).

He doesn't do or say anything, he just watches, and if the players go up to him and try to interact, he just marks a notch on the table in front of him with a hook.

The joke is that he's the guy who hands out plot hooks, but every time the party has tried to interact they've already gotten one, and he won't give a plot hook to somebody who already has an adventure. As of yet none of the group have ever interacted with him when they genuinely don't have a plot to follow, and we're pushing into multiple years of gaming together.

In every story I've ever written or theorized (not even purely for tabletop), this character named Shinobu exists.

He's a ninja. Not a very bright one, but extremely over confident and boastful, and has a constant need to be validated of his greatness. In my current campaign he's an associate of my PCs of sorts, tags along with them on some of the dicier quests if they need clutch combat support but most of the time he just sulks and broods.

Sometimes he goes rogue and tries to sabotage the party's interests, but he usually fails. They haven't kicked him to the side of the road yet despite all that, since he's something of a fan favorite NPC and they find him really funny.

If it's any sort of modern non-fantasy setting Shinobu is generally a the leading character of some blockbuster in-game movie franchise, and has very little relevance to the plot.

An oracle named Victoria Clemens. She shows up at random points and gets friendly with the party, no matter what game we're playing. Real nice lady, can't keep a secret for the life of her. Lot of fun.

I constantly reuse the same rival for the PCs. He has a few quirks I always keep around, a sweet-tooth and a fascination with foreign culture, the name, but I play him differently in every incarnation. He's always introduced in roughly the same way and my players are split on their opinions to him, one hates him and wishes I'd stop bringing him in, one hates him but loves hating him and sometimes lets me bridge him into the game through their character, one adores the idea and gets excited wondering what I'll do with him each game, and one of them really doesn't seem to care at all and just treats him like a new encounter every game.

He's never been a major antagonist though and I'm still hoping I'll have a chance to make him the last encounter someday.

Yes.
He has a special power.

What power?

An extremely rude half-dragon librarian. It's L-space, I don't have to explain shit.

Regardless of setting or system, I'll always try to sneak in an old farmer who took up arms and leads a surprisingly effective militia. He's bitter that he's had to delay his retirement, and often complains about his creaky bones, astray children, and adventurers all being vagabonds and thieves.

Well my campaign ended up being a sequel to another game my group played because I made a joke so the party has met one of the old PCs who was a half-elf bard. In the first game he was an asshole that basically manipulated the barbarian into doing his bidding,lied to every NPC to get what the party needed and when the adventure was over he left without a word. He had become a wandering bard for a bit then got shot gunned wedding'd to a half-orc that he pissed off during the game. He ran a bard college and just grew to dislike adventurers because whenever they come to town someone's going to break his shit.