• ZenWalk was unique and great about 15 years ago as an easy Slackware with minimalist install.
• Chakra Linux was an Arch+KDEmod distro that kind of went away.
• Bodhi Linux has its own desktop called Moksha.
• There is a GNUstep Live CD that comes out every few years, based on Debian. It is a unique setup from a time when the future of computing was promising. I think it is distributed on LinuxQuestions or some other forum.
• There was a distro called gOS about 15 years ago that used a lot of desktop widgets and Google apps. Their business model was basically, "We are going to re-skin Ubuntu and call it gOS and hope Google buys us." It did not work out.
• Darwin was upstream for macOS and for many years, there was a community of users who would port the traditional *NIX stack to it. Xorg, traditional window managers, a ports system, etc.
• Frugalware Linux was well polished and kind of a spiritual successor to Zenwalk.
• openSUSE 10.3 had the most beautiful Gnome setup. It was unique in that it had a single panel, a modified Clearlooks theme, and a Vista-style start menu.
• OpenSolaris likewise had a very unique and beautiful look, with its macOS-inspired Nimbus theme. I think this was the best looking theme of that era.
• SimplyMEPIS was my first Linux on a T61. I had used FreeBSD for the decade prior. I don't know what was better about SimplyMEPIS than Debian, nor do I know what SimplyMEPIS meant versus regular MEPIS. It's kind of like Claws Mail and Sylpheed Claws. Some times we just throw words together and give it an icon and there it is.

I used all of these at some point.