Nowadays processors are so tiny and cheap, they're everywhere.

# batteries

IIRC most battery charging circuits also have a dedicated real time ~OS running. http://www.youtube.com/watch?v=dlSBQ5b6Pdw‎

# hard drives

Also recently someone did run linux in its hard drive controller (which is a set of arm cores, ~v9 and m3)

HaD intro : http://hackaday.com/2013/08/02/sprite_tm-ohm2013-talk-hackin...

Direct link : http://spritesmods.com/?art=hddhack