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.
There are a few aspects we need to consider…
For the UI, I would go with beOS, Haiku is cool, but the look is a bit too modern, I love the clean look of beOS’s UI
For the best compabillity, after a lot of work, ReactOS, a project aiming to create an open source os that is binary compatible with Windows 2000.