Two player simple Tetris built with Elixir Phoenix.
I based this game off the code and slides from Josh Adams' ElixirEU 2015 conference talk http://www.elixirconf.eu/elixirconf2015/josh-adams.
From that starting point I added that the down key and two-player mode.
Get a buddy and two different computers. Each of you go to https://tetris-jc.herokuapp.com/.
Hopefully your game is on the left side, and your friend's game in on the right. (And vice verse for your friend on their screen.)
But there's a chance that you didn't get matched with each other to play head to head, and if that's the case have your friend refresh and then you refresh.