CP/M
Dominant operating system for 8-bit microcomputers 1977–1983. Gary Kildall, Digital Research. Direct predecessor to MS-DOS (which IBM chose after a famously missed meeting).
Stack: BIOS + BDOS + CCP. ~250 KB filesystem volumes on floppy disks. Major machines: Osborne 1, Kaypro II, IBM Displaywriter. The IBM PC story: when IBM came in 1980 to license CP/M-86, Kildall was out flying, his wife refused to sign the NDA → IBM went to Microsoft → MS-DOS (bought from Seattle Computer Products for $50k) became a billion-dollar industry. CP/M died quickly after the PC took over.