Other OS Next to GNU/Linux, there are some other 'Linux'-alike (it's better to say NIX-alike) distributions.
I don't say that you can start 'out of the box' with them if you know GNU/Linux, but they
deserve a fair chance.
Amoeba is a powerful microkernel-based system that turns a collection of workstations or single-board
computers into a transparent distributed system. It has been in use in academia, industry,
and government for about 5 years. It runs on the SPARC (Sun4c and Sun4m), the 386/486, 68030,
and Sun 3/50 and Sun 3/60.
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
This emphasize portability, standardization, correctness, proactive security and integrated cryptography.
OpenBSD supports binary emulation of most programs from SVR4 (Solaris), FreeBSD, Linux, BSD/OS, SunOS and HP-UX.
FreeBSD is an advanced operating system for Intel ia32 compatible, DEC Alpha, and PC-98 architectures.
It is derived from BSD UNIX, the version of UNIX developed at the University of California, Berkeley.
It is developed and maintained by a large team of individuals. Additional platforms are in various stages
NetBSD is a free, secure, and highly portable UNIX-like operating system available for many platforms,
from 64-bit AlphaServers and desktop systems to handheld and embedded devices. Its clean design and
advanced features make it excellent in both production and research environments, and it is user-supported with
complete source. Many applications are easily available.
Apple's open source projects allow developers to customize and enhance key Apple software. Through
the open source model, Apple engineers and the open source community collaborate to create better, faster
and more reliable products for our users.
I'm under the impression that the BeOS-development has stopped.
The BlueOS project consist of the creation of a modern OS from the ground up.