Modders and brothers MrBean35000vr and Chadderz banded together and created a mod that allows for online play of New Super Mario Bros. Wii. Ultimately, the mod is aimed at adding online capability to any local-area multiplayer game on the Wii. The project started as a mod for New Super Mario Bros. Wii specifically and grew to fit a more general purpose goal, though as there are still some kinks to iron out in New Super Mario, they have not tested it with any other titles.
For those interested, the process works by “sending a long stream of inputs over the Internet; button presses on a controller. An assigned host dictates RNG (random number generator, random elements in games like which item you get from a box in MKW for instance), and synchronises level loads so that everyone plays the game at the same speed. It also has an adjustable lag threshold, to allow smoother gameplay with people far away with you, at the cost of slight input delay.”
It is not perfect, yet, to be sure, as there are still desynchs the occasional missed input, but it is an impressive project nonetheless.