I periodically make games and they periodically have multiplayer in them.
These are the various assets and examples that I've made. Most are for GameMaker. See also: My blog ! It has tutorials. HTML5-specific extensions for GameMaker
These are the extensions that solve variously exotic problems that you may encounter while making HTML5 games in GameMaker. See also: My blog ! It has tutorials. Non-HTML5 assets and example
Games with online multiplayer support that I have worked on.