Hello.
TeeWorlds is stupid, retro 2D shooter, not requiring instalation of it´s client. It is open-source C++ application, easily modifiable, and relatively lightweight, with easy server settings. Server is only one .exe taking less than 1MB. Both server and client are availible for Windows, Linux and Mac.
https://www.teeworlds.com/I used to play TeeWorlds before, even had my own fansite at
TeeWorlds.cz, now at
TeeWorlds.cz archive. I guess we could get some players on, with CTF gamemode and ctf5 map. However, I doubt it could bring any money to CG - there is nothing you would really use.
However, there are certain gamemodes made into TeeWorlds - some of them could get some $$$ from donations.