The Pi can also be done as bare metal (someday I need to try it).
http://www.valvers.com/open-software/raspberry-pi/step01-bare-metal-programming-in-cpt1/
Interesting stuff, but also somewhat painful, perhaps someone could take those ideas and craft a bare metal Arduino core for the Pi hardware.
Another option that is gaining traction is MicroPython. It is sort of halfway between bare metal and an OS. I have been following a fork call CircuitPython. When you modify and compile MicroPython that is done at the bare metal level, but when it is running Python programs it is somewhat more like an OS, or perhaps it could be seen as a patchwork of bare metal functions that give the illusion of being an OS.