SkullRush

Source (GitHub)

If I had to sum up SkullRush, I'd say it's a 2D online multiplayer platformer shooter. Yeah, it's that cool.

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, custom maps, custom weapons, custom game entities, custom 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, and since the game is open-source, the modding SDK is… the source code! SkullRush is licensed under GPLv3 (some things it depends on are licensed otherwise, but yeah the source code is GPLv3).