Nintendo WFC will shut down in May 2014

[Qaazavaca Qaanic]

I think CTGPR (I do not recommend) and Wiimm's newest Mario Kart Fun 2014-04 both have support. Wiimms Mario Kart Fun does not need a registered friend code to run (tested in Dolphin), but almost nobody plays it online. CTGPR, I think they modified the game in a way that removes the requirement to preregister the friend code, but I'm not completely certain.