Game Server Control Panel
https://play.battlesnake.com/ – You play by coding snake movement.
Multiplayer Game Servers
https://github.com/heroiclabs/nakama – Nakama is the leading open source game server framework for building online multiplayer games in Godot, Unity, Unreal Engine, MonoGame, LibGDX, Defold, Cocos2d, Phaser, Macroquad and more.
Photon Server – 100 clients free. – require Windows server to run.
OpenTibia – Open source game server for game Tibia. Works on Linux/windows.