Xen, Minix 3, QNX, several l4 implementations, etc. OKL4 alone has been shipped on over 1.5 billion devices. QNX was huge. Xen is running everything at AWS, the largest webhosting company in existence (nevermind everywhere else it is used).
QNX is huge. If you took QNX out of the world it would stop to function just about immediately. So much stuff runs on it that you typically wouldn't even guess. In that sense it is just like other soft real time/hard real time OS's one of the unsung success stories of IT simply because it works so well it tends to disappear.
Most things that use QNX or other OS's like that inside simply work rather than that they require constant upgrades and bug fixes. Reliability by design is so much better than reliability by trial and error, and the OS is a huge factor in that. It's a world where bugs are felt as 'egg on your face' rather than a 'wontfix'.
