There's always Valheim...

But I host Titanfall servers. People are able to play on them using the Northstar client, a mod for the game that lets you connect to community servers, as well as mod the game in general.

Minecraft, Veloren, Valheim, Core Keeper

I've not played it in years, but Soldat was always great fun. Server requirements are pretty much: potato.

Skyrim together, if you're particularly into jank.

Tradewars fucking 2002.

Yes, I said it. I stand by it.

Terraria. It's a sort of hybrid between a minecraft 2d and an action RPG, really fun game with lots of content.

You can play it as standalone or as co-op and the server component is included in the game for you to host it if you want.

Tho If by interesting you mean MMOs, there are emulators for a few of them, WoW is my favorite (TrinityCore, AzerothCore), but know that opening them up to public use is illegal (copyright violation).

Edit: added github links to wow emulators.

Project Zomboid is worth looking at. It's not my kind of game, but it's great at what it is (top down zombie survival crafring)

There's some reverse engineering port of WoW server, for example TrinityCore

Core Keeper

If you don't like minecradt you probably won't like these 2 much either but I'll leave it here for anyone who might be interested anyways

Gmod and brickrigs (basically a sandbox Lego game but you have access to a massive community made workshop)

To further my little mention about Runescape though I meant classic I believe there are efforts with the modern-day one: https://github.com/Open-RSC/Core-Framework

You can also host final fantasy 11 servers: https://github.com/LandSandBoat/server

Maybe I'm old, but I loved hosting Open Arena, Teeworlds and Cube 2: Sauerbraten.

Space Engineers is really fun. Kind of a cross between Minecraft and Factorio, but in space.

These are ones I keep coming back to: Farming Simulator, Satisfactory, and Valheim. I've also kept up a V-Rising server at times because my friends like it, but I'm not as into that game. Always on the lookout for games to host for friends, so I'm glad I stumbled upon this thread! Found a few in here I'm going to try out.

Garry's mod. Doesn't use a lot of resources (compared to newer games like Satisfactory), and you have an almost unlimited amount of content and things to do because of the Community Workshop.