The Nintendo Wi-Fi Connection (WFC) service was an online gaming service that provided free online play in various Nintendo DS and Wii games. Nintendo discontinued the service on May 20th, 2014, primarily because the host of the service, GameSpy, was shutting down its servers. Nintendo WFC was required for online play, but the Wiimmfi Project has replicated much of its functionality and allows for continued online play in Mario Kart Wii and other games.

With a bit of patching, this allows many Nintendo Wii and DS games’ online functionality to work again. The project doesn’t seem to have any updates since 2021 but the servers are still online and active.
Depending on the game, it might even be possible to get it working again without patching thanks to Kaeru’s project.