The past week or so I made a multiplayer pong game using Lidgren.Network library for XNA, I call it "MythoLogic Pong". All you have to do is to connect to an opponent's IP. It also works over Hamachi.
You can download and install the game completely free from MythoLogic Interactive.
I made this game in order to learn how Lidgren.Network works and how to create a multiplayer game (which I have never done before) and how to optimize the data packages being sent. All this to prepare for Superfighters Deluxe.