We will of course mention SkyOS which seems very promising indeed, Clicker32 (multitasking open-source operating system project, based on a modular micro kernel and a brand-new architecture), RDOS (fast, multi-threaded OS for PCs and 386+ based embedded systems, runs many DOS, DPMI and Win32 console programs, 100% assembly with segment protection), ModulOS (an OS for the i386 architecture, where each device driver, file system, etc, is a module that can be loaded and unloaded at run-time from the system), ZealOS (another OS with a focus on being modular), OppcOS (an orthogonal persistant OS which means that it preserves its entire system status across system reboots, replacing the concept of file systems for persistancy), Odin (high performance, assembler coded, both desktop and server OS), Cerafix (a fast, secure, reliable, and easy to use OS for x86 and later PowerPC processors), ROME (a modular, multitasking, embedded operating system. The system is highly modular, with functionality split between multiple processes), GazOS (an open-source OS written as an example for budding systems programmers), MenuetOS (a Real-Time, graphical OS written in 100% 32-bit x86 assembly, distributed under the GPL), Gemini Nucleus (a production quality micro kernel based on the latest findings in OS research), ChaOS (a micro-kernel OS), Brix (a graphical OS), Mobius (32-bit monolithic kernel design for i486 and higher), GeekOS (a simple but realistic example of a tiny OS kernel running on real hardware) and Flamethrower OS (Flamethrower is an OS initiative that aspires to become a fully functional, easy to use desktop and server operating system with elements of OSX, Plan9 and BeOS).

If you know more OSes that are part of the niche, but very active and they mean... somewhat serious business, let us know by posting a comment below.