Post games here that are free and have multiplayer features
https://www.beyondallreason.info/ - RTS game that plays somewhat like Command and Conquer, still in development, but it plays very good and is fairly optimized, it will be open sourced when it is complete and allow for modding
https://xonotic.org/ - FPS, Open source Quake-like shooter, runs great, lots of modding abilities
Plutonium - FPS, Modded versions of Call of Duty with multiplayer and zombies, you will have to find torrent links for both of the games though for this to work, use archive.org on the Plutonium pages for working magnets
Team Fortress 2 - Team Fortress 2
League of Legends - Not DOTA
Unturned - FPS, Similar to Day Z, has Minecraft-esque graphics, very simple to play and runs on everything
War Thunder - Tank/Plane/Boat shooter with a focus on realism, fairly grindy though