The Nintendo Wi-Fi Connection service, which allowed online gaming on both Wii and DS, was shut off on May 20 this past year. Since then, various modders have worked to bring some of that functionality back. MrBean35000vr and Chadderz are two such modders, and their first target is a game that never had online multiplayer to begin with: New Super Mario Bros. Wii!
You can see the magic in action in the video above. There are several problems at the moment preventing the mod from being released, such as accelerometer movements not transferring, players desynchronizing when a star is acquired, and support for only two consoles. So far, however, the project looks phenomenal!
The best part about this mod is that it can be easily adapted to any other game. MrBean35000vr and Chadderz plan to release the source code so that others can build upon it and apply it to other titles. There's even potential for online play with GameCube, WiiWare, and Virtual Console games!