Solar OS
Solar OS is a one to few person project. An OS with a GUI with an early 90s feeling. Closed source, if I didn't miss something. There is an API reference, but not much help beyond that. I didn't see a software repository mentioned anywhere. I guess the most likely usage is with your own custom applications or ported software.
Solar OS's advantages are definitely its small size (< 1.44 MB) and how fast it runs (a 386 with at least 8 MB RAM should be enough). I booted it from a USB flash drive (using grub and Syslinux) on an Athlong 64 X2. It was as fast as it could be, of course. Although, movement of the mouse pointer is very laggy with screen resolutions higher than VGA. With HD resolutions it can't really be called usable with a mouse anymore. I don't know if that's a driver issue with my graphics hardware or what. This is not a problem with other systems that use a simple VESA driver.
But, there wasn't much to do to test how well, fast, comfortable, beautiful and error-free everything runs. There aren't many applications included. The things that are included do a good job of demonstrating the OS. There are things like a process manager, a picture viewer, a very simple text editor and other tiny demo and system applications. And there is a help decument, debugger, things like this. For a proper test, I'd wish for more applications to use for practical work. I don't know what exactly. It depends on what you need the os for. Maybe writing your own application would be best included in a test that aims to collect for experience about the OS.
It crashed after less than half an hour trying it out. Because it stopped responding before I made any screenshots and I didn't yet bother to boot it up again, see the screenshots page on the project web site. There are many screenshots that paint a good overall picture of the OS and its GUI.