RxDOS - Industry-strength DOS compatible RTOS used mostly in embedded systems and similar uses.
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, IEC1131, more.
TSX-32 Operating System - General purpose, multi-user, multi-tasking, DOS compatible OS, written from the ground up for Intel platforms. Real time support. Embedded device support.