If we go with first/third person open world RPG, then there really aren't that many games like it.
I guess you can count Legends of Valor, which was a major influence on The Elder Scrolls Arena, but it's a very limited game compared to TES. It has some interesting ideas though. Not that open a world since it's set in a single city, no globetrotting wilderness wanderings here.
Then of course Ultima Underworld 1 & 2 which started the whole "first person exploration RPG with free movement" genre. It's not an open world but a big multi-level dungeon, which can be explored in any way you wish. Highly recommended. Lots of cool shit to discover in this game.
In the vein of Ultima Underworld, there's also Arx Fatalis. Big multi-level dungeon with lots of fun stuff to discover. But like UU it lacks an overworld.
And uh... I guess we can count Ultima IX too. It's third person, it has an open world, the issue is just that it sucks as a game. Only play it if you're desperate.
Gothic, Risen, Elex... pretty much every single game Piranha Bytes ever made is an open world exploration-focused RPG. Their worlds are quite a bit smaller than TES on average (with ELEX and Gothic 3 having the biggest worlds) but they're of high quality. Lots of stuff to discover and a decent hand-crafted difficulty curve. No random stuff here. They're top notch and the only thing on the market that even remotely compares to Elder Scrolls. Sadly it lacks custom character creation. Risen 2 and 3 are less good than the rest, though (and less open world than the rest).
Kingdom Come Deliverance is a "Skyrim killer" set in medieval Bohemia. Others already recommended it - it's good. And one of the very very few historical RPGs out there.
Two Worlds is a budget version of Elder Scrolls. It's decent, but nothing special. Two Worlds 2 is more of the same but the world is less open, instead it's divided into distinct zones, each of which is its own mini-open world. It's not great, but it's decent enough. There are also several expansions for it because the developer decided to milk it dry... I don't know if those expansions are any good, didn't try them, but the most recent one has a shit review score on Steam.
Outward, as others already mentioned, is a game that tries merging TES style exploration with survival game elements. It's pretty decent overall but the world feels a bit empty.
Enderal was already mentioned. Do try it out, and also give Nehrim a try - a total converison mod for Oblivion, made by the same team as Enderal. Both are available as standalone installs on Steam, free to play if you own the base game on Steam.
I guess Might and Magic 6 to 9 kinda somewhat sorta fit into that mold. You play as a party of 4 characters instead of a single dude/dudette, but the exploration is pretty great. There's lots to discover, there's a spell that allows you to walk on water, a spell that allows you to jump very high, a spell that allows you to fly... lots of fun stuff.
There's a cheap indie RPG I picked up on the last Steam sale called "The Dark: Survival RPG" which kinda fits the bill. It has more survival elements than TES, but it also has a big handcrafted world to explore with stuff to find in every corner. Haven't played it a lot yet though so I can't give a definite "Is it worth playing?" answer.
D.W. Bradley's Dungeon Lords is a third person action RPG with an open world and exploration, but its structure is way more linear than TES, and it's overall a very mediocre game. Not even half as great as the man's previous works.
Fallout 4 is surprisingly decent if you ignore the story and focus on exploring the dungeons. It has some genuinely good dungeon design, which is a surprise after Skyrim's donut-shaped dungeons of linearity.
I guess the Dark Souls games kinda somewhat qualify? They're open world, you have full custom character creation, and they're action RPGs with finding loot and leveling up your stats. They feel very different to TES, but they're the same basic genre of open world exploration-focused action RPG.
Assassins' Creed Odyssey and Valhalla both added "RPG elements" to the old Assassins' Creed formula, but to be honest the introduction of these elements made the games worse rather than better. Many enemies are HP sponges and you have to grind repetitive side quest activities to level up. Meh.
Dragon's Dogma was already mentioned, and it's pretty damn good. The only action RPG where companions are an actual help rather than getting in the way of your main char. Sadly the cities feel a bit dead because it's a Japanese-made game and Japs don't know how to deliver interactive NPC dialogue and sidequests.
FRONTIERS is a kickstarted game that promised to be a spiritual successor to Daggerfall, with more focus on exploration and mapping out the world than fighting combats. People had high hopes for it, but the solo dev had some issues with managing a project this big, and the end result was very disappointing. But credit where credit is due, he didn't abandon the game but finished it and released a working 1.0 version. It's just nowhere near what has been promised in the Kickstarter. Play it if you're really desperate for more games of this type.
The Precursors is a Russki-made slavjank FPS-RPG hybrid in a sci-fi setting with some beautifully weird vistas. It's light on the RPG elements but it's a pretty good game regardless. You get several planets to explore, and even space battles to fight in a figher shuttle. The planets are all open worlds, but they're relatively small, and the first planet is the most content-packed one. They become more sparse as the game goes on.