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Free Operating Systems

Syllable
According to the sourceforge download page: Syllable is a complete Operating System, with its own kernel, filesystem, GUI and applications. Syllable is based on the AtheOS codebase, is largely POSIX.1 compliant, and uses a large number of the GNU utilities and applications.

MenuetOS
This is a hobby OS for the PC. It is written in 64-bit assembly and is no longer open source, but you can still download the 32 bit sources.

PC BSD
PC-BSD has as its goals to be an easy to install and use desktop OS, which is built on the FreeBSD operating system. To accomplish this, it currently has a graphical installation, which will enable even UNIX novices to easily install and get it running. It will also come with KDE pre-built, so that the desktop can be used immediately.

SkyOS
SkyOS is an operating system written from scratch for x86 PC's. As required for a modern operating system, it supports features such as multiprocessor support (SMP), virtual memory, memory protection, multitasking and threading etc. Additionally, SkyOS has a built-in GUI system named SkyGI.

NewOS
NewOS is a free, full-featured operating system with full source code available, along with periodic binary snapshots.

FreeBSD
FreeBSD is an advanced BSD UNIX operating system for the Intel compatible (x86), DEC Alpha, and PC-98 architectures.

NetBSD
NetBSD is a free, secure, and highly portable UNIX-like operating system available for many platforms, from 64-bit AlphaServers to handheld devices.

OpenBSD
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.

Linux
Linux is a free Unix-type operating system developed under the GPL license.

FreeDOS
FreeDOS aims to be a complete, free, 100% MS-DOS compatible operating system. Beta 6 is available.

FreeDOS32
A 32-bit operating system under development for the i386 platform, designed to extend the DOS concepts to a native 32-bit environment, in order to offer a better support to modern 32-bit DOS protected mode applications.

Minix
A free Unix clone that is designed to be easy to learn.

Atheos
AtheOS is a free desktop operating system under the GPL lisence. AtheOS currently run on Intel, AMD and other compatible processors and support the Intel Multi Processor architecture.

Plan 9 From Bell Labs
The third release of the Plan 9 operating system from Bell Labs.

FreeBSD Java Project
This is the offical port of Sun's Java Development Kit for FreeBSD.