QNX - from 1982, a microkernel OS. (And Amiga...)

QNX has been very widely deployed, perhaps most visibly in the ICON thin clients from Burroughs and later UNISYS:

(As mentioned here earlier: Ontario’s Computer: The Burroughs ICON)

There’s a paper on the internals of QNX recently discussed on HN:
An Architectural Overview of QNX by Dan Hildebrand (PDF, 16 pages)

… a realtime microkernel surrounded by a collection of optional processes that provide POSIX- and UNIX-compatible system services. By including or excluding various resource managers at run time, QNX can be scaled down for ROM-based embedded systems, or scaled up to encompass hundreds of processors…

A short related blog with more links:

Oh, and the Amiga connection: during the time Gateway owned the Amiga portfolio, they contracted QNX to deliver the OS… and QNX didn’t deliver.