For example Project Cartographer describes itself as the following
Project Cartographer is a modification for Halo 2 PC (Halo 2 for Windows Vista) that restores online play (Microsoft abandoned the LIVE servers) and adds additional features that are not present in the base game.
Before the release of Master Chief Collection it was my go-to for playing online.
Similarly H1-Mod exists for CoD: Modern Warfare Remastered.
I'd be interested to hear which ones you've tried and how active the community is. They don't need to be currently active or a replacement for a game's existing multiplayer services. It could be something like Zandronum which allows you to play the classic Doom or NVMP-X which allows you to play Fallout New Vegas with friends.
Most of these services don't contain DRM so you can use Steam, GOG, Humble Bundle, or any other version of the game and because they are community run you don't have to worry about servers going down as time goes on and can potentially host your own games.
I also like to make a special mention of X Link Kai which allows you to play system link games remotely. If you are curious I'd check out ConsoleMods.org for setup instructions and their website for a list of supported games.
There's plenty of other similar services out there depending on what console you are using such as Insignia for the original Xbox.
I'd love to see these communities grow.
I've spent more time than I'd like to admit playing EQ on Imperium.
Super mario 64 multiplayer! I have spent several hours in it with my bf. https://github.com/djoslin0/sm64ex-coop/
There's Nitrox for Subnautica, but development seems to have stopped for now with no ETA on continuing. There are bugs and it's recommended to avoid certain building structures and riding in the Cyclops together. Still, it can be really fun
Thug Pro for Tony Hawk's Underground 2.
Plutonium for multiple older Call of Duty games.
IW4x for Modern Warfare 2 (2009), IW4x was DMCA'd semi recently but the mod still works, it's just no longer being maintained and the server browser has a few bugs.
Even for games I've bought off Steam I still use community run multiplayer mods a lot because they tend to either add more functionality or replace dead servers for older games.
GamesRanger is a pretty dope RTS client for a ton of RTS games. I use it for Battle for Middle Earth 2 to this day.