<A HREF="http://www.drdos.com/"
  >DR DOS</A> (or DR-DOS) is an *(OS), a free-for-non-commercial-use version of _(DOS); was once released as OpenDOS, but Caldera reverted to the historical name. <A HREF="http://www.caldera.com"
  >Caldera Systems, Inc.</A> (now <A HREF="http://www.sco.com/"
  >SCO</A>, they acquired it and subsequently reverted the company name to SCO) does no more <A HREF="http://www.caldera.com/company/drdos.html"
  >own</A> it.

It's basically the very stable and complete implementation of _(DOS) by Digital Research, built off CP/M-86 (see a little known history of why <A HREF="http://www.novell.com/"
  >Novell</A> bought it "<A HREF="http://www.ctyme.com/dri.htm"
  >Digital Research - The Untold Story</A>", by Marc Perkel), with some networking and multitasking capability, ROMability, etc. 
Sources for the _(kernel) have been released, but the free software community that gathered around that event (OpenDOS _(FAQ) by Alaric B. Williams <A HREF="http://www.delorie.com/opendos/faq/"
  >here</A> and <A HREF="http://www.deltasoft.com/faq.html"
  >there</A>) felt betrayed because Caldera did not make the system free of rights, and did not release all the sources after all. Go _(FreeDOS) instead!

Caldera also sue the well known Seattle-based company for unfair competition practice in forcing people and vendors not to buy from competitors, at the time DR-DOS was technologically way ahead.
