OS/2
OS/2 lost because IBM had to have things done their way, be that good or bad. Gates didn't want to take it in the direction IBM wanted to go, so they parted ways.(And part of that direction was that OS/2 be rock solid and reliable. IOW, Enterprise class. Windows, at that point in time, was not. Some would still argue this point but that's for another thread and has been beaten to death.) IBM has accepted (I say accepted and not learned) this with Linux. I think they accept the Linux situation partly because it is making money for them.