Eduke32 - https://eduke32.com/

OpenTTD - https://openttd.org/

Commander Genius - https://github.com/gerstrong/Commander-Genius

Open Lara - https://github.com/XProger/OpenLara/releases

Open Jazz - https://github.com/AlisterT/openjazz

City of Heroes - https://cityofheroes.dev/

Scummvm - https://www.scummvm.org/downloads

Red Nukem - https://github.com/nukeykt/NRedneck/releases

OpenRA - https://github.com/OpenRA/OpenRA/releases

SDLPoP - https://github.com/NagyD/SDLPoP

Among others.

Dunno if this is exactly the kind of thing you have in mind, but a person known as 'Yuvi' has been singlehandedly maintaining publicly available servers for the original Demon's Souls for about 4 years (the official servers shut down about 5 years ago).

Thanks to Yuvi, you can still play the original game with all online functionality, regardless of whether you're playing on PS3 or RPCS3.

There is forged alliance forever . It revived supreme commander 1

Discovery Freelancer is a multiplayer Freelancer mod. I've only discovered it today, but I'm already a couple of hours in.

Freespace 2 Open, the games source was released in the early 2000's under a ... dubious licence that may or may not be legal to work with but no one really knows and it's been more than 20 years so eh

Freespace 2 far and away my favourite space game, nothing else comes close to the tension of war that game setup so it's great that you don't have to run a .exe from the 90s to play the game. lots of mods for it now too thanks to this project

CorsixTH - Theme Hospital

OpenRCT2 - Rollercoaster Tycoon 2

RVGL - Re-Volt

Rollcage

Rollcage 2

The Zod Engine - Z

There's OpenGOAL which makes the Jak and Daxter games run on modern PCs natively. AFAIK only the first game works flawlessly so far. They're such great games, I'm glad someone is going for it.

Someone also ported Mario64 to PC a while ago. You need to jump through some hoops to get it yourself, or sail the high seas to find an exe file that's already compiled.

Oh you have found a rabbit hole now.. enjoy! :)

DevilutionX Diablo I

OpenMW Elder Scrolls III: Morrowind

Mineclone 2 Minecraft

• Ultima - Graphics patch
• Ultima II - Upgrade patch
• Ultima III - Upgrade patch
• Ultima IV - xu4 or the upgrade patch, but actually the Sega Master System version is probably the best official version
• Ultima V - Upgrade patch
• Ultima VI - ScummVM
• Ultima VII - Exult
• Ultima VIII - ScummVM

Prince of Persia 1 - SDLPoP

Sonic in 3rd person - Sonic Robo Blast 2

I see you've mentioned Zdoom, I"ve sank many hours into Brutal Doom (Extermination Day), Blade of Agony and Golden Doom. Found the Hakros Launcher recently which opened some doors (though not the most up to date, still a decent gateway).

There's also OpenMW a recompilation project of Morrowind.