Optiboot (an older version) is installed by default on the Arduino Uno and (as of 2018) official Arduino Nano boards. It can be installed on all older mega8, 168 or 328 based Arduinos. As of 2019, ...

QP/C Real-Time Event Framework/RTOS implements event-driven Active Object (Actor) model combined with Hierarchical State Machines. Specifically designed for embedded systems (e.g., ARM Cortex-M MCUs).