Penny-DOS, The FOSS DOS.

Jemm386 and JemmEx are socalled EMMs ( Expanded Memory Managers ) or, more exactly, V86 monitors.

A 54 bytes snake game in assembly

A minimalist host environment for flat memory model 32-bit DOS applications

A 32-channel MOD player for DOS

MOD player library for PMI

Programs written in x86 Assembly for DOS.

Shared API for DOS and OS/2 to produce dual mode applications

ALP/MASM compatible macro library for BIOS and DOS

Home of the HX DOS Extender and its DPMI-host HDPMI.

A set of DPMI debuggers for DOS and 16-bit Windows

Seattle Computer Products 8086 Monitor ROM collection

MS-DOS 1.25 source code

MS-DOS 2.11 source code

The Touhou PC-98 Restoration Project

A modern, scratch-built, open-source (GPLv3) alternative ROM BIOS for PC, XT, 8088 Clone or Turbo PCs.

DOS Assembly Programs and Utilities

A tiny Brainfuck interpreter

Enhanced DR-DOS kernel and command interpreter ported to JWasm and OpenWatcom C

Typeable DOS quine