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.
For microcontroller MSP430, ARM, AVR, PIC18, HCS12 and other.
FreeRTOS.orgTM V4.1.2 now available: Includes new ports and demos for:
Microchip PIC24
Microchip dsPIC® DCS
STMicroelectronics STR750 (ARM7).
Link: https://www.freertos.org/