Syllable Desktop is a complete operating system that is easy to use, powerful, lightweight and extraordinarily fast and responsive. It has its own kernel, filesystem, GUI and applications. Syllable Server is a matching small and efficient Linux server.
Haiku is an open-source operating system currently in development designed from the ground up for desktop computing.
FreeOS/1 is a Free, Open Source, FreeDOS based Free and Open Source Alternative OS, with open source Microsoft Windows compatibility using HXDOSExtender KernelEx,and ReactOS source code. FreeOS/1 will include new technologies and will be easy to use
A fork of the QEMU emulation system to allow all people to run multiple OS systems from a main system drive or partition such that they can provide their OS machines with whatever degree of direct hardware access or emulated hardware access needed.
open source analogue of IBM OS/2 operating system