I actively use the following general purpose operating systems:
- Linux, GNU or otherwise
- OpenBSD
- Solaris
- DOS

I have used these systems in the past, and am still comfortable with them:
- Haiku
- plan9
- FreeBSD
- Windows NT
- DOS-based Windows
- macOS 10
- ROOL

I am familiar with these embedded or networking operating systems:
- Cisco IOS
- Ubiquiti Vyatta
- Ubiquiti Debian
- Mikrotik routerOS
- pfSense
- GRUB2 shell