Sort of. Like everyone else, I’m not quite sure what you’re looking for. However, some free operating system/distributions that are different in various ways:

• Haiku. Already mentioned, it’s the least like what you’d expect from Linux. It’s a clone of BeOS, an also-ran from the 90’s. Actively developed.
• Redox OS, a micro kernel written in Rust. Very early, probably rough around the edges.
• 9front. Bravely carrying the Plan9 torch, probably the most actively developed Plan9-ish variant.
• Chimera Linux. The Linux kernel, but without Poetteringware (no systemd, no pulseaudio), and using the BSD userspace instead of GNU. Not to be confused with Chimera, a Linux distribution targeting the gaming crowd.
• MINIX3. One of the most famous micro kernels, and famously related to Linux by the fact that its inventor and maintainer is a professor who Linus Torvalds studied under, before Linus went and created Linux.
• BSD (Free…, Open…, there are, like, a half dozen variations. Not Linux; derived from a Unix. Also not GNU userspace - I think they’ve got a sysv userspace?

These are all open source and free, and actively developed. Some are young; some are old and perpetually never finished; some are stubbornly standing still. Most are probably usable as daily drivers, with some constraints.

Where the Linux kernel leads the pack, by a mile, is the vast amount of hardware or supports, and except for the Linux kernel distributions like Chimera Linux (the only one in my list that uses the Linux kernel), hardware support is likely where you’ll find the most challenges.

Of these, the first three are the only ones that don’t use X11 or Wayland, much less the Linux kernel. The last three use X. None use the GNU user space.

There are a bunch of other projects, several based on the L4 microkernel, but these are the ones I think you could boot onto from a USB stick and actually use. 9Front might have the steepest initial learning curve; Haiku is probably the most new-user friendly. MINIX and 9Front are the most geeky.