Where can I find open source code for RTOS?
RTEMS - http://www.rtems.org/ftp/pub/rtems/4.9.3/
FreeRTOS - http://sourceforge.net/projects/freertos/files/FreeRTOS/
eCos - http://ecos.sourceware.org/getstart.html
Trampoline - http://trampoline.rts-software.org/spip.php?article77
I used μC / OS-II and it is very good. I myself have not used Contiki.
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Zephyr rtos from intel: https://www.zephyrproject.org/downloads
NUTtx RTOS: http://nuttx.org/doku.php?id=downloads