Programs that compile to run directly on the computer hardware without an explicit OperatingSystem (though I guess they have a kind of LibraryOS compiled in with them)
- BareMetalRaspi for things that compile to bare metal on the RaspberryPi
- BareMetalSmalltalk for SmallTalks built on bare metal
- LispMachines for equivalent for LispLanguage
- Arduino and similar SingleBoardComputers are often without OS.