Cemu is the first Wii U emulator released, in October 2015. Cemu is current under active development, and making significant progress. It is capable of outputting Mario Kart 8 at around 60 FPS, and Super Mario Maker at 60 FPS. Synchronized sound is only partially supported in a few games, and visual accuracy for most games is poor. It doesn't yet support Virtual Console emulation.
Decaf is another Wii U emulator under active development.