If I remember well, Trenitalia POS and many European banks still use OS/2. A lot of mainframes have OS/2 as a frontend to boot (IPL) the big iron. Got heartbroken when OS/2 got sold.
I was amazed the first time I saw one of these crashed, showing the OS/2 Logo (Warp 3 or 4 I think). From the GUI it was impossible to tell, they had relatively high resolution and even, at some point, videos embedded.
I wonder who developed and updated the GUI, and using what.
For more recent systems (IBM mainframes, Sun/SGI large machines, etc.) the controller also handles things like power sequencing, hardware configuration (e.g. physical partitioning of interconnect buses, resource allocation...) and mainly also enforces the hardware licensing policies.