6 OPERATING SYSTEMS YOU PROBABLY NEVER HEARD OF

1. ReactOS

ReactOS is an open source computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up (2000, XP ).  ReactOS is primarily written in C. This is not a Linux based system, and shares none of the unix architecture.
 

Web: www.reactos.org

2. Haiku

Haiku is a  free and open source operating system compatible with BeOS currently in development that specifically targets personal computing. Inspired by the Be Operating System, Haiku aims to become a fast, efficient, simple to use, easy to learn and yet very powerful system for computer users of all levels.

Web: www.haiku-os.org

3. Minix 3

MINIX 3 is a  open-source operating system designed to be highly reliable, flexible, and secure. It is loosely based somewhat on previous versions of MINIX, but is fundamentally different in many key ways. MINIX 1 and 2 were intended as teaching tools; MINIX 3 adds the new goal of being usable as a serious system on resource-limited and embedded computers and for applications requiring high reliability. It is published under a BSD license and is a successor project to the earlier MINIX 1 and MINIX 2 operating systems.
Web: www.minix3.org

4. Syllable

Syllable Desktop is a free and open source operating system for Pentium and compatible processors. Its purpose is to create an easy-to-use desktop operating system for the home and small office user. It was forked from the stagnant AtheOS in July 2002.

Web: www.syllable.org

5. KolibriOS

KolibriOS is a small open source x86 operating system written completely in assembly. It was forked off from MenuetOS in 2004 and run under independent development since. The KolibriOS team has managed to squeeze pre-emptive multitasking, a graphical desktop, TCP/IP networking, USB support, media playback and many other features onto a 2.85 MB

Web: www.kolibrios.org

6.JNode

JNode (Java New Operating System Design Effort) is a free software project to create a Java platform operating system. The project has taken the unique direction of creating all the software in Java itself, with the exception of some assembly language to boot and load the system JNode uses only two languages, Java and assembly.

Web: www.jnode.org