Don’t expect them to have plenty of apps, much less professional ones.

Haiku OS, a BeOS Clone  
https://www.haiku-os.org/

Icaros, an Amiga Desktop clone for 64 bit x86 computers
http://icarosdesktop.org/

Redox OS
https://www.redox-os.org/

KolibriOS
http://kolibrios.org/

RISCO OS OPEN
https://www.riscosopen.org/