From Rosetta Code

BeOS is a single-user multitasking graphical operating system, originally targeted at the BeBox computer, and later adding PowerPC Macintoshes and the Intel x86 architecture. Although it sees little use today, it still retains a small but loyal following.

A few OS's were created that were inspired by BeOS (or were direct or indirect continuations of it) including magnussoft Zeta and BlueEyedOS. The only one that is still in active development is Haiku.