TurboDOS was a contemporary one - had multiprocessing (as in, you could have multiple Z80 CPU boards in a single card cage that passed messages over a bus) and was delivered as linkable objects, so you could customize the OS for your hardware (in the multi-Z80 setup, you didn't need any I/O that wasn't in the daughterboards, so in this case the tiny version of TurboDOS on the daughter boards did the message passing thing and talked to the physical serial ports, but didn't need a disk driver since the only disk was hooked to the single master board. Great (long-lost) stuff - we were building an early dialup info-service so each daughter board had multiple modems...