Free Operating Systems

• AIOS
  The AIOS project is aimed toward developing a new computer operating system based on artificial intelligence.
• BPMK
• FreeBSD
  FreeBSD is one of several free, monolithic BSD 4.4-lite derivative operating systems. It provides full UNIX support, including networking, X Windows, and almost all other normal UNIX services.
• HURD (Free Software Foundation GNU Project)
  Related to: Mach
  The HURD is the operating system being developed by the Free Software Foundation as the basis for the GNU Project, which has already produced such well known tools as Emacs and GCC. The Hurd is a personality for the Mach micro-kernel which exports a bevy of services to the application. The Hurd will provide the standard UNIX interface, but should also be much more flexible than standard UNIX.
• Linux
  Group Members: Linus Torvalds and a Cast of Thousands
  Linux is a freely-distributable implementation of UNIX for 80386, 80486 and Pentium machines. It supports a wide range of software, including X Windows, Emacs, TCP/IP networking (including SLIP/PPP/ISDN), and the works. Ports to non-x86 machines such as the Alpha and SPARC also exist. This is one rocking project.
• Lites
  Related to: Mach
  Lites is a 4.4 BSD Lite based server and emulation library that provides free unix functionality to a Mach based system. Lites provides binary compatibility with 4.4 BSD. NetBSD (0.8, 0.9, and 1.0), FreeBSD (1.1.5 and 2.0), 386BSD, UX (4.3BSD) and Linux on the i386 platform. It has also been ported to the pc532, and PA-RISC. Preliminary ports to the R3000 and Alpha processors have also been made.
• NetBSD Group Members
  NetBSD is one of two free, monolithic BSD 4.4-lite derivative operating systems. It provides full UNIX support, including networking, X Windows, and almost all other normal UNIX services. Unlike many other free UNIX implementations, NetBSD has also been ported to a large variety of hardware platforms.
• OpenBSD
  Yet another group pushing a free BSD-Lite derivative. This group splintered off of the NetBSD project, for reasons unclear to me, although I'd guess perhaps becuase NetBSD wasn't pushing out regular releases fast enough, and FreeBSD wasn't supporting non-x86 platforms. The splintering of BSD people has effectively killed any chance of BSD beating Linux, expecially given the good work porting Linux to other platforms.
• RTX
  Group Members: Mike Podanoffsky
  RTX is a very small, very fast real time executive that utilizes signals and queuing as a basis for managing and scheduling tasks. It becomes very easy to support multiple processors, communication channels, and to synchronize processes. RTX is completely free, but it is not public-domain software. If you decide to use the software, you may receive an automatic license to do so even in commercial products, if you provide adequate, reasonable credit to its developer.
• VSTa
  VSTa is an experimental kernel which attempts to blend the design of a micro-kernel with the system organization of Plan 9. The result is a small privileged kernel running user-mode tasks to provide system services such as device drivers, file systems, and name registry. Like Plan 9, each service provides a file system-like interface.