Wikipedia article covering the history and design of DOS.
Doctor DOS Betamax; DOS Operating System
Commentary, tutorials, references, Power-Users' Corner, for DOS and related utilities.
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]
A barebones DOS alternative.
Weblog covering useful tips for a number of command line interfaces including the DOS shell.
Unofficial PTS-DOS FAQ
In English and German languages; with wiki, downloads, links.