I love this community, got so many recomendations I decided to update the first post! Sorry if I couldn't reply to everyone, Jerboa almost exploded, haha. But I'll upvote all your contributions now that I'm on PC.
Original post
I like to play on the same computer with my bf, and recently we've started to get tired of MAME since we've been playing our faves on repeat for so long.
The other game we play is Spelunky, since is the only local co-op game I have on Steam. I ask because Steam summer sales are close and I can give him a surprise :D
Wow!!! I had completely forgotten about this site. What a fantastic resource. Does anyone remember, was this page started by the Penny Arcade guys or did they just plug it in the beginning?
It turns games that don't have a native local coop mode into local coop by simulating a LAN and running two instances of the game communicating exclusively with each other. It works really well, especially if you have a multi-monitor setup.
I have played multiple games in local coop with my brother with this, including Elden Ring. It's been a blast.
Holy shit does that thing have functionality. Elden Ring split screen with only one copy? Deep Rock Galactic split screen? Dark Souls 1 and 3? Sign me the fuck up!
Yes, you only need one copy of the game for most of the games supported by Nucleus Coop. It creates its own simulated Steam users/accounts for the game in the background.
It Takes Two is absolutely awesome if you don't mind platforming. The game director made a boast/bet that if anyone found the game boring he'd pay $1000. I'm not getting that $1000.
It's one of the few games I bought within a year after launch in the past decade and I have to say I didn't regret spending the money.
Halo MCC unfortunately doesn't have splitscreen support on PC unless you use a mod to run the game twice and connect the two instances via online (maybe also LAN) co-op.
I wouldn't recommend A Hat in Time, the local co-op is really bad because of how the camera works. :(
A few steam games I know:
ibb & obb
Dinasty Warriors 8
We played on PS but also available for pc:
It Takes Two
Stardew Valley
Children of Morta
Divinity: Original Sin 2
Cuphead
Guacamelee
Moon Hunters
Out of Space
I was a bit confused when you said local co-op. It implies 2 computers on the same local network(connected to the same router, no internet connection required). I think the term you're looking for is split screen gaming.
When I looked at pictures of Spelunky games like Clonk and Liero popped into mind, I think there are options for split screen multiplayer in those games, they're old school PvP games.
Magicka - top-down mage play where you can combine spells with each other (two beams make a stronger beam, etc), and friendly fire if you want to occasionally nudge or troll each other
Trine - side-scroller in 3D graphics with 3 unique characters; some platforming, action, and slight puzzles, and a very good story presentation/direction
Would you and your BF enjoy shooting at alien scum while your characters scream DEMOCRACY!! and HAVE A NICE CUP OF LIBERTEA!! at the top of their lungs? Would you be entertained by accidentally killing each other in the heat of battle when you radio for an ammo drop?
Then Helldivers would be for you. Join the forces of Super Earth and spread Managed Democracy throughout the universe!
PROTIP: If you want to assert dominance in your relationship, find a gun with a bayonet attachment. You will mix up the melee and interact buttons on at least one occasion.
Also, Helldivers 2 has been announced. So not the worst time to get into the first gem! Though, split screen support has not been confirmed for the sequel, and it may not make an appearance since they're making the jump to a 3rd person perspective. But it still looks good, and I am fanboying over it.
Many PC games support online co-op but have no splitscreen support. For a lot of these you can use a mod to run two instances at the same time and play them in splitscreen. Here's a link to the supported games: https://hub.splitscreen.me/
They've already been mentioned, but the two greatest games I've played with my SO is "It takes two" and "Unraveled 2". Please do yourselves a favor and play those!
Recently picked up PlateUp, can definitely recommend that one as well.
It's like a roguelike Overcooked. If your relationship survives this surely you can handle it all
Remember if you're a PC gamer, emulation is a great place to look for couch co-op since PC gaming seems to be rather averse to split screen unfortunately. There are a lot of extremely good split-screen games on PS2/Dolphin that will look and run significantly nicer on PC.
Not real co-op, but my wife and I have played hundreds of hours of Wildermyth. Its an rpg with generative elements. We love reading out the lines of the characters in terrible accents. The battlr system is turn based so you've plenty of time to discuss what to do next.
Unravel (So touching, made me cry)
Project Zomboid (One of my fav games. I've played this split screen with my son. He kills the zombies, I organise the base)
A Way Out (Prison escape story)
Pinball FX3 (A classic two player experience)
Stardew Valley (Crafting/farming)
Beyond Two Souls (Story)
Skullgirls (2D Anime platform fighter)
My wife and I really loved these two that almost no one talks about since they haven't been around for long:
Rogue Heroes: Ruins of Tasos (co-op Legend of Zelda!)
Sun Haven (similar to Stardew valley)
I'd recommend LEGO City Undercover as well. Imagine LEGO GTA and you've pretty much got the gist. Very fun playing with my wife on the Switch, but I'm sure it's got a much better framerate on PC.
Gang Beasts is ludicrous fun! Flailing around, grabbing your little clayboy opponent and yeeting them off the side of a lighthouse will never not be fun. Definitely best played with a controller, though.
If you want a diablo-esque ARPG check out Ghostlore. It has some really cool mechanics with the glyphs, skill grid, and food. Oddly enough, the only multiplayer it supports is local co-op.
Edit: It should be noted that I haven't tried the co-op yet and after looking at the steam page again there's a review saying that it's buggy, so take this recommendation with caution.
Dariusburst Chronicle Saviors is one of my favorites. It's a horizontally scrolling shmup with massive bosses and multiple routes. It has ultrawide support too.
The DLC is single-player only, so if you just want co-op, just get the base game
Unrailed is a hectic but fun small game about building tracks in front of a train in motion. Lots of bumping into each other and panicking, if you're into overcooked-like time based games