A well-known company in Seattle has produced and mismaintained the most crappy *(OS) ever, known as Disk _(OS) or DOS. The dreaded name need not be mentioned here.

Of course, there exist lots of DOS emulators for lots of non-DOS systems. 
For instance, _(Linux) has quite a good free DOS emulator running on i386 architectures (also runs under *_(BSD)). But it is actually a hardware emulator, and requires that you use an actual _(DOS) software (e.g. _(DR DOS) or _(FreeDOS)) to run on top of the virtualized hardware. Newer versions of <A HREF="http://www.dosemu.org/"
  >DOSEMU</A> will be released as a bundle with _(FreeDOS).
