FreeRTOS.orgTM is a portable, open source, mini Real Time Kernel – a free to download and royalty free RTOS that can be used in commercial applications
FreeRTOS.org Features:

* Free RTOS kernel – both preemptive and cooperative options.
* Designed to be small, simple and easy to use.
* Supports both tasks and co-routines.
* Free development tools (Cortex-M3, ARM7, MSP430, H8/S, AMD, AVR and 8051 ports)
* Free embedded software source code
* Royalty free
* Sample pre-configured complete embedded solutions – an easy way to get started
* Cross development from a standard Windows host
* Demo applications for selected single board comput