sird.ps by Stephen Peters (speters@us.oracle.com) is a random-dot
stereogram in only 603 bytes (plus header)!

BEST OBFUSCATED ARTWORK -- 2nd prize
  -- The 2nd most coveted prize. These combine obfuscation with great artwork.

The sird.ps program produces an output which looks like a large rectangle,
randomly filled with dots, and two large circles at the top.  This is called a
random-dot stereogram, and has occasionally been referred to as a sird.  In
order to view the image, cross your eyes until the two circles at the top merge
into one (it should now look as if there are three circles at the top).  Now,
focus on the center dot, and your brain should start interpreting the dots as
part of a 3-D picture.  It will be as if part of the field of dots is "warped"
toward or away from you.
 
If you can't focus on the central dot, try focusing first on objects close to
you or far away from you until you get the right effect.  Don't feel bad if you
can't get it at first -- it's something that takes practice!
 
If this peaks your interest, try Games magazine from (around) April '92 --
there's an article on the creation, and several example sirds to look at!