I make the specification of non-linux because otherwise this would just become a thread full of obscure distros that do the same thing as a million other distros.

Some lesser known OSs:

• AROS - based on Amiga OS, has some derivatives like IcarOS and MorphOS
• Haiku - based on BeOS
• Redox - Unix-like, made in Rust (might technically count as linux?)
• Serenity - Unix-like, very late 90s look and feel
• Kolibri - Tiny OS, the image is ~44MB. It also has a smaller version that fits in a single floppy.
• PhantomOS - When 3 Russians decide to turn everything about a typical OS upside down.