DOS
   Wikipedia article covering the history and design of DOS.
   http://en.wikipedia.org/wiki/DOS

Doctor DOS Betamax; DOS Operating System
   Commentary, tutorials, references, Power-Users' Corner, for 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/

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/

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/

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

Shell Tips
   Blog covering useful tips for a number of command line interfaces including the DOS shell.
   http://www.shell-tips.com/category/dos-windows-batch-files/

Unofficial PTS-DOS FAQ
   In English and German languages; with wiki, downloads, links.
   http://www.almnet.de/ptsdos/index.en.html

ZDOS
   Realtime, 32-bit replacement for 16-bit DOS, with modern features: multitasking, demand paging, SMP. Coded in pure Assembly language. Zebor Technology.
   http://www.zebor.com/main/zdos.html