Decaf Emulator is the only currently actively developed, open-source Nintendo Wii U Emulator. It is the open source alternative to the excellent proprietary Wii U emulator, CEMU.
While Decaf doesn’t emulate the Wii U as good as CEMU at the moment, it is an extremely promising start and is already making big strides to catch up to CEMU in emulation capabilities.
Initially, Decaf emulator was a Windows 10 only Wii U emulator, and this was due to its initial use of DirectX 12. However, the primary developer of Decaf, Exjam, made the decision to drop DirectX 12 in favor of OpenGL 4.5. This change allows Decaf emulator to be multi-platform.
Decaf is already able to run some commercial Wii U games, so it’s great if you’re looking to play Wii U games. However, at this moment, you are likely to run into an assortment of graphical issues and severe bugs. Decaf is very much in the experimental stage, but Decaf has made quick progress in just a few short months.