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 have used a certain “online-fix me” (not sure if I’m allowed to promote) website that offers patches for games so you can connect to the official multiplayer servers.
Of course there’s a possibility of getting banned if they detect it, but I’ve played a lot of the Forza games online with this throughout the years without issue.
I realise it’s not strictly on topic for community run servers, but it may be of interest to people in this thread.