QNX is fascinating on its own, but MINIX 3 is still a different project in that its full adoption of a NetBSD userland will probably make it more useful for generic servers and workstations as well. They also seem to be going much deeper with checkpointing and dynamic upgrades/hot code reloading.
If you need hard real time and the problem is too big for something like VxWorks, QNX is still the way to go.
There's all sorts of much tinier RTOS like FreeRTOS, MicroC/OS and Contiki that are used out there for particularly critical and/or constrained environments.
QNX has a big advantage in that from userland it's basically a Unix. You can develop on it completely self-hosted on a desktop PC. The GUI's pretty good; it even comes with Java and Eclipse.
They've downplayed that recently, alas, but I believe that if you hunt around on their website you can find a bootable CD. I think platform support has slipped a bit so you might have trouble making it boot.
Way back when, there was a QNX demo floppy, which was a bootable 1.44 MB floppy disk which contained a full GUI, web browser, dialup modem support, etc. It'd run on a 386 with 8MB of RAM.
If you need hard real time and the problem is too big for something like VxWorks, QNX is still the way to go.
There's all sorts of much tinier RTOS like FreeRTOS, MicroC/OS and Contiki that are used out there for particularly critical and/or constrained environments.