The MachOS project is a reference build of the original CMU Mach kernel distribution and documentation for development of new distributed computing platforms that require integrated IPC and multi-processor support. The project includes the kernel source distributions and a GNU based cross compile build chain.
Visopsys is an alternative operating system for PC compatible computers. Under development since late 1997, this system is small, fast, and open source. It features a simple but functional graphical interface, real preemptive multitasking, and virtual memory. Though it attempts to be compatible in a number of ways, Visopsys is not a clone of any other operating system. You can demo the distribution from a "live" CD or floppy disk.
ReactOS is an advanced operating system providing a ground-up implementation of a Microsoft Windows XP/Windows Server 2003-compatible operating system. It aims to achieve complete binary compatibility with both applications and device drivers meant for XP and NT 5.2 operating systems by using a similar architecture and providing a complete and equivalent public interface. Written completely from scratch, ReactOS is not a Linux based system, and shares none of the UNIX architecture.
amforth is an extendible command interpreter for the Atmel AVR ATmega microcontroller family. It has a turnkey feature for embedded use as well. It does not depend on a host application. The command language is an almost compatible ANS94 forth with extensions. It needs less than 8KB code memory for the base system. It is written in assembly language and forth itself.
MirBSD originated as a patch set against OpenBSD-current, an ultra secure operating system and NetBSD derivate, and has since also incorporated changes from NetBSD, a 4.4BSD-derived ultra portable operating system. It features bugfixes, code removal for the sake of simplicity, and feature enhancements over stock OpenBSD as well as a much more up-to-date GNU toolchain, careful integration of patches from other projects (such as KAME), and many improvements. It works on the Intel Pentium and some 80486 machines with more than 32 MiB RAM and the SPARC, and a port to the PowerPC Macintosh is in preparation.