Haiku is an open-source operating system currently in development designed from the ground up for desktop computing.
Just Another "X" Object Oriented Operating System An embedded operating system aiming to take advantage of modern object oriented and generic programing.
Simple OS made for educational purposes. Not preffered for a desktop OS. Made for developers.
A FOSS operating system that aims to meet NSA's TPEP TCB A1 evaluation standard while retaining application compatibility at minimal performance overhead.
A small multitasking OS to run on 65C816 based systems. Features to include a GUI support for as much HW for the supported systems as possible and a system of system extensions for easy expandability of the OS. The initial Development is being done on the Apple IIgs, with plans to port to the Commodore 64 with a SuperCPU. Eventually there will be ports to other 8 and 16-bit CPUs beginning with the 6502, followed by the z80, 6800, 6809, 8086, PDP-11, and then the TMS9900. Once these are complete there will be 32-bit ports to the 680x0, ARM, MIPS, etc.