Vimtronner

Vimtronner from Carbon Five is an online multiplayer game for learning Vim. It requires Node, and the controls are entirely based around Vim’s commands.

You can start a game by pressing i, and the movement keys are hjkl. It's run as a command-line tool which accepts switches for things like listing games, and running a local server.

The server is basically a web app that uses WebSockets, so you could technically install it on a service like Heroku.