RealDOS  
   A barebones DOS alternative.
   http://www.realdos.de/english/

Turkish Rational DOS  
   The core of DOS compatible operating system that is currently under development.
   http://www.singlix.com/trdos/index.html

Paul Edwards's Programming Page  
   Contains PDOS, a public domain DOS alternative.
   http://freespace.virgin.net/paul.edwards3/program.htm

Idiots Operating System  
   A floppy-disk based operating systems distribution with a focus on multimedia applications.
   http://shernoam.port5.com/index.html

ROM-DOS  
   Full embedded OS; compact, reliable, fully compatible DOS for embedded uses, mobile computing environments, fast connecting of embedded systems to Internet. By Datalight, Inc.
   http://www.datalight.com/product_detail.asp?p_id=9&archive=0

IBM Corp.  
   Makes PC DOS 2000; was one of the two original DOSes for the IBM PC. Now year 2000 compliant, and updated for a new century. Preserve your investment in DOS applications.
   http://www-3.ibm.com/software/os/dos/

PC DOS for Embedded Devices  
   Retooling of IBM PC DOS for use in embedded systems.
   http://www-3.ibm.com/software/os/dos/dosemb/

RMF-DOS  
   Reduced Memory Footprint; goals: match functions of MS-DOS 6.22, IBM PC DOS 7.00, DR-DOS 7 subset. Written mainly in 2 free C compilers and assembler, most parts are smaller uncompressed than compressed parts of MS, IBM, DR. [Open Source, BSD]
   http://sourceforge.net/projects/rmfdos/

PhysTechSoft Ltd.  
   Makes PTS-DOS 2000: network support, file manager, archiver, internal boot virus protection, graphical web browser; email, telnet, FTP clients; many utilities, some for embedded system development.
   http://www.phystechsoft.com/ptsdos/

Dos/Windows commands since v1.00  
   An index of all built-in commands available in all versions of MS-DOS. Includes description and syntax of commands.
   http://home.earthlink.net/~rlively/MANUALS/INDEX.HTM

Doctor DOS Betamax Power-Users' Corner  
   Commentary, tutorials, and references for MS-DOS and related utilities.
   http://www.chebucto.ns.ca/~ak621/DOS/DOS-Head.html

Georgos the DOS compatible operating system  
   A free DOS-compatible operating system. Site includes sample applications, discussion forum, and development information.
   http://www.georgos.org/

The unofficial PTS-DOS FAQ  
   In German and in English language. Also wiki, downloads, and links.
   http://www.almnet.de/ptsdos/index.en.html

ZDOS  
   A 32-bit multitasking, real time replacement for MS-DOS.
   http://www.zebor.com/main/zdos.html