Vampire OS is the alternative to Windows and macOS
freeRTOS for Arduino Uno, Goldilocks 1284p & Mega 2560
OS bootable from Linux,Windows and others partitions as auxiliary OS
amforth is an extendible interpreter on microcontrollers
OnePlus Nord (Avicii) Builds of PSM-Kernel
The smallest and the most powerful operating system.
Gobudgie is an operating system which is compatible to Android.
OS-9/6809 compatible operating system for Tandy CoCo and Dragon
Secure file transfer with SSH