DOS

Concurrent Controls, Inc.
Makes, sells, supports Multiuser DOS, Concurrent DOS; helps businesses and VARs do more with less.
http://www.conctrls.com/

Multiuser DOS
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/Multiuser_DOS

On Time Informatik GmbH
Makes, sells, supports RTKernel, realtime multitasking system for DOS; compact (about 16k code, 6k data); gives programmers the basic tools needed to develop efficient realtime software. No royalty.
http://www.on-time.com/rtkernel-dos.htm

S&H Computer Systems, Inc.
Makes TSX-32 general purpose, multiuser, multitasking, text-based, non-graphical, DOS compatible RTOS, coded for x86 platforms. Supports realtime use, embedded devices.
http://www.sandh.com/os.htm

Technosoftware, Inc.
RTXDOS-16 (DOS compatible) and RTXDOS-32 (WIN32 compatible) real-time OSs for embedded PCs. Modular systems with TCP/IP, WEB server, Process IO, OPC Toolkit, and IEC1131.
http://www.technosoftware.com/