While Nintendo has officially killed off online functionality for the DS and Wii, the internet being the internet, people have found a way to keep it alive. Sort of. For now.
Private servers and custom code are keeping DLC downloads and some multiplayer services alive, though it's important to note that's for a limited range of titles (though Mario Kart and Advance Wars are working), and you're going to need homebrew-capable hardware for it to run.
This isn't a permanent solution, but if you need to squeeze a few more days/weeks/months out of your last-gen online (and aren't afraid to mess with your consoles), have at it!