ReactOS is a pretty interesting GPL-licensed, non-UNIX OS from my testing. :ina_nod:

It's a reverse-engineered implementation of the Windows NT kernel to be binary-compatible with Window binaries. The project has been around since 1998, and has been in alpha for a while.

It's only 32bit and often breaks a lot so its not yet ready for daily driving but the team behind ReactOS has been collaborating with the Wine devs to deal with compatibility so kudos. :awoo_thumbs3: