BeOS

BeOS screenshot
[ This picture copied from www.digibarn.com under Creative Commons License. ]

IBM OS/2 was the apex of PC operating-systems -- until BeOS was ported to PC. BeOS was mainly influenced by MacOS and to some extent Amiga OS.

In ~1999, dual Pentium II mainboards became available. BeOS was the only (or best) operating-system written for multi-processor PC systems. (Rare MP version of OS/2 may have existed. Linux 2.x MP was badly done by locking entire kernel.) What this author remembers most about BeOS is that it was sleek, booted fast, ran fast, never crashed.

This author viewed BeOS as an advanced UNIX-compatible system with MacOS GUI. Its file-system was journaled and had some database functionality. Although BeOS had GCC and core GNU commands, it had enough differences that most open-source software wouldn't compile.