DOS

Commands Reference with Versions, Syntax, Notes, and Examples
Index, description, and syntax, of all built-in commands available in all versions of DOS/Windows since v1.00.
http://www.geocities.com/rick_lively/MANUALS/INDEX.HTM

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

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

Georgos the DOS compatible operating system
A free DOS-compatible operating system. 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/

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

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/

Shell Tips
Weblog 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