SkullRush

Source (GitHub)

SkullRush is a 2D online multiplayer platformer shooter.

The game is dead-simple: open the client, connect to a server, and blast stuff. That's all I can guarantee, because mod support is huge, and mods can change the game upside down. In short, the client acts as a canvas of sorts, and the server has almost complete control over the game's graphics, sounds, and logic. This means that there's out of the box support for custom game modes, maps, weapons, game entities, graphics (that can even override the default game graphics if you want) and much more. In fact, the official game modes are written as mods themselves.