OS-9 is a real-time operating system originally developed by Microware in the late 1970's to take advantage of Motorola's powerful new 6809E microprocessor. OS-9 had a concept of modules: all programs and OS components were modules, and could be loaded dynamically. Everything was re-entrant, using a link-count like that of the UNIX filesystem. The I/O system was broken into several layers using an approach that was object-oriented. OS-9 was the first program that I ever considered beautiful.

They had things like interlaced GIF display while downloading years before anybody else did.

I had the pleasure of working with OS-9's successor, OS-9000 (I'm not sure what the differences were between OS-9 and OS-9000 other than a larger memory area and more modern underlying memory management).

OS-9 also ran/runs in various forms on the Atari ST and I kept hearing about an Amiga port but never actually saw the beast.