Originally released on the 28th of June 1994, FreeDOS is now 15 years old. FreeDOS is a free open source DOS clone. The current release, version 1.0, was released in early September of 2006 and is licensed under GPL.

Even though (Free-) DOS itself was superseded by operating systems with graphical user interfaces (GUIs) in the mid-nineties, it's still a great example of an open source equivalent for a proprietary operating system.

more here and here