Updated: 12/29/2005
OS
Amiga OS
Clone of Amiga 3.1 OS. [Report a dead link]
http://freshmeat.net/projects/aros/?topic_id=864%2C144

Athena OS , 1,480kb
Object oriented full OS. Requirements: 32MB ram, 3MB disk space. This is not a shell-replacement or file manager, it is a full OS. [Report a dead link]
http://www.rocklyte.com/downloads.html

DrDos
Also at drdos.org. [Report a dead link]
www.drdos.com

Freedos (WIP)
Free DOS operating system. [Report a dead link]
www.freedos.org

Freedos 32 (WIP)
[Report a dead link]
http://freedos-32.sourceforge.net/

Freeos
Listing of free operating systems, mostly Linux-like. [Report a dead link]
http://www.freeos.com/

MoeDos
Another DOS clone. [Report a dead link]
http://www.moedos.com