Free Operating Systems
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.
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 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 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 is a free, full-featured operating system with full source code available, along with periodic binary snapshots.
FreeBSD is an advanced BSD UNIX operating system for the Intel compatible (x86), DEC Alpha, and PC-98 architectures.
NetBSD is a free, secure, and highly portable UNIX-like operating system available for many platforms, from 64-bit AlphaServers to handheld devices.
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
Linux is a free Unix-type operating system developed under the GPL license.
FreeDOS aims to be a complete, free, 100% MS-DOS compatible operating system. Beta 6 is available.
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.
A free Unix clone that is designed to be easy to learn.
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.