I was a big BeOS fan in the mid-to-late 90s and ran it on a hand-built dual P2-300 computer.  I don’t know how many CPUs it supported back then, but its architecture built around pervasive multi-threading (so as to take advantage of multiple CPU cores) hasn’t been completely forgotten in this multi-core world:

Ars Technica: Microsoft exec: Future versions of Windows to be “fundamentally redesigned”

Back in 1991 when ex-Apple executive Jean-Louis Gassée was starting development of a new operating system called BeOS, its designers tried to make the entire operating system “pervasively multithreaded” in anticipation that multiple CPUs would be much more common in the future. This ensured that any one window that became unresponsive would not interfere with any other windows, although the forced multithreaded programming model increased the risk of programming errors such as race conditions and deadlocks.