The Freescale MQX Real-Time Operating System (RTOS) provides real-time performance within a small, configurable footprint. The RTOS is tightly integrated with the latest 32-bit MCUs and MPUs from Freescale and is provided with commonly used device drivers

FunkOS is an RTOS for 8-32 bit MCUs including AVR, MSP430, and Cortex M3. Both C and C++ kernels avaiable. Key features include preemption, periodic events, device drivers, and mutex/semaphores. Includes a variety of graphics and filesystem middleware.

The Fusion Embedded RTOS (Real-Time Operating System) from Unicoi Systems is a Real-Time OS designed and optimized for networking and media-centric processors.

Mark3 is a sophisticated, modern RTOS and application development platform, targeted towards a growing list of today’s most compelling embedded devices.

Nut/OS is an intentionally simple RTOS for the ATmega128, which provides a minimum of services to run Nut/Net, the TCP/IP stack.

Apache NuttX is a real-time operating system (RTOS) emphasizing standards compliance (POSIX, ANSI) and a small footprint. Highly scalable, it operates on microcontrollers from 8-bit to 64-bit architectures. NuttX provides a rich, multi-threaded development environment suitable for deeply embedded systems. Its architecture includes a preemptible core, extensive networking capabilities (including IPv6 and 6LoWPAN), various filesystems, USB host/device support, a graphical subsystem, and options for protected builds using MPU/MMU. Licensed under Apache 2.0, it fosters an open development environment supported by GNU toolchains and simulation capabilities.

RT-Thread is an open-source, community-driven embedded real-time operating system (RTOS) established in 2006, primarily targeting IoT and diverse embedded applications. It offers both a standard version for feature-rich devices and a lightweight Nano version requiring minimal resources (3KB Flash, 1.2KB RAM). RT-Thread features a modular architecture with a real-time kernel, extensive middleware components (like filesystems, networking, GUI), and a rich package ecosystem. It emphasizes ease of porting across various MCU architectures (ARM, RISC-V, MIPS, etc.) and provides development tools like RT-Thread Studio and Env for configuration and building.

RTAI is the RealTime Application Interface for Linux - which lets you write applications with strict timing constraints for your favourite operating system. The RTAI distribution includes RTAI-Lab, a tool chain to convert block diagrams into RTAI executab

RTEMS is an open source RTOS that supports open standard application programming interfaces such as POSIX. It is used in space flight, medical, networking and many more embedded devices.

StratifyOS is a powerful embedded RTOS for the ARM Cortex M microcontrollers.

Xenomai is a real-time development framework cooperating with the Linux kernel, in order to provide a pervasive, interface-agnostic, hard real-time support to user-space applications, seamlessly integrated into the GNU/Linux environment.