I was going to mention QNX Demo Disk in my UNIX alternatives comment. I think I edited it out for a weak fit to the post. It was an amazing demo, though, showing what a clean-slate, alternative, RTOS architecture could do for a desktop experience. The lack of lag in many user-facing operations was by itself a significant experience. Showed that all the freezes and huge slow-downs that were "to be expected" on normal OS's weren't necessary at all. Just bad design.

It's neat that it was the thing that inspired one of your Squeak projects. Is SqueakNOS usable day-to-day in any console desktop or server appliance context? Key stuff reliable yet?