I learned C on QNX (back then, it booted from a floppy on a PC/XT). It was a nice little Unix-like OS, with all the things you'd expect from a nice little Unix-like OS, plus a reputation of being rock-solid like nothing else.

I think it's a real shame Blackberry didn't manage to etch a third (or fourth - I also loved Palm's WebOS) niche for their QNX-based phones. Blackbberry 10 was an amazing mobile OS.