ToaruOS
This one I consider one of the more interesting ones. It has a modern, well-design desktop and windows manager. It's user-friendly in that it just start as expected with no need for settings or choices and has an introduction for newcomers. The well-rounded look makes it look very mature. In fact, the first time I booted it I checked that it really isn't "just" a Linux distribution that's trying to hide that fact on the surface. And it isn't. The person who developed ToaruOS over 13 years apparently focused on finishing the part that help create a good first impression before starting other features that might be expected from an OS.
The one feature that makes ToaruOS stand out among other hobbyist operating systems is one of its compositor: Windows can be rotated in any angle by dragging the window's title bar while holding down the ALT key.
Besides the usual UNIX-like command line utilities there are very few apps pre-installed on the live system. Not even a graphical text editor. There is a package manager. But since the NIC of my laptop is not supported, I couldn't try it out.
Overall this feels like a very promising project. An OS that is usable right now and would benefit the most by getting more apps written for or ported to. ALl tho more disappointing that the sole developer has abandoned the project recently.