Some uITRON code is free, but more are not free. The uTRON I used is coded by the man in my company followed the uITRON Reference V3. You can find the code in some magazine and change the a litter to fit you CPU.
I think, RTOS has been applying in industrial equipmets to control or supervise circle lines. I also know RTOS had been a components in military devices.
We need RTOS when we don't wish to get into the nitty gitty details of the underlying hardware. We may let RTOS handle that part and allow overselves concentrate on our specific task/application.
This usually also allows us the portability and provides us with extra services that we may use to simplify our application development.