FreeRTOS is described as 'real-time operating system kernel for embedded devices that has been ported to 35 microcontrollers. It is distributed under the GPL with an additional restriction and optional exception' and is an app in the Development category. There are three alternatives to FreeRTOS for Windows, BSD, Linux and Mac. The best alternative is Zephyr, which is both free and Open Source. Other great apps like FreeRTOS are NuttX (Free, Open Source) and embOs (Freemium).
NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 32-bit microcontroller environments, the primary governing standards in NuttX are Posix and ANSI standards.
embedded Operating System (embOS), is a Real-time operating system, developed by the Company SEGGER Microcontroller. It's designed to be used as a foundation for the development of embedded real-time applications for a wide range of µc.