Tribblix is an open source operating system created by Peter Tribble. Based on illumos, it blends a retro style with modern components.
NEWS: September 11th 2023: Milestone 32 available. This is a functional release. (updates). Vanilla Tribblix and LX (aka "omnitribblix") variants for x86 are now available. (Release Notes.)
NEWS: August 2nd 2023: Milestone 27 ISO available for SPARC. (download - Release Notes.)
Important: 32-bit support.
While the SPARC version still suffers from lack of testing and has spotty hardware support, the x86 version is pretty solid. You can download the image, install to a hard drive, and use it.