Richard Barry's FreeRTOS made a compact preemptive real-time kernel freely available across many microcontroller families, becoming reusable infrastructure for embedded scheduling and synchronization.
The Zephyr Project brought a small real-time operating system for constrained devices under neutral Linux Foundation governance, combining multi-architecture portability with open development, security processes and modern embedded tooling.
VxWorks brought priority-driven real-time scheduling, networking and cross-development tools into a durable commercial platform for deeply embedded systems.