StellaAn OS by Tony Tebby, one of the fathers of modern personal computers whose fame mostly comes from having created the Sinclair QL (Quantum Leap) computer and its MC68000-based, multitasking, graphical operating system in the early 80's, even before the Amiga. Stella's base principles are componentisation and scheduling optimisation for single-processor computers (that perform fake multitasking).
From its home page (see below):
Stella is a new real time operating system. It runs on 680x0 processors and may be adapted to ColdFire processors. Although designed for embeded systems, it could be easily extended to run on any computer system.