For the Global Game Jam 2016, I made a game called Altar. You can play it by clicking the banner on this post.
The theme of the jam was "ritual". In Altar, you run around a 2D sidescroller as one of many (random) characters and try to push people off the edge of little platforms. It was a big hit at the game jam, I'm proud to say, and many people were playing it, using glitches to fly to the hidden base (hint: top right of the map) and generally suggesting new concepts for the game.
Part of the jam is uploading your source code online, so I'm doing that here.
Essentially there are two parts to the game: The client and the server.
The server must be hosted on player.io, a service that provides net code for multiplayer games.