ELIZA is a famous dialog program written in 1966 by Joseph Weizenbaum at MIT.
Reference:

   Weizenbaum, J., "ELIZA -- A computer program for the study of natural
   language communication between man and machine", Communications of the
   ACM 9(1):36-45, 1966.
 
This is a fast upload I did on Jan 20. 1999 to satisfy students and collegues.
It contains several versions of ELIZA. Use them at your own risk.
All programs are copyright their copyright holders. Except for the basic
 

DOS-based ELIZA

which I re-tested (runs well in a Win95 window), I am not sure they even work.

So here are the others. Note that the Net is full with unreliable or modified versions.
And here is a BASIC source code of 256 lines. Just read it, even if you know nothing
about  programming; most of it is in plain English. You see how simple a thing it is.

For more of classic AI software, see e.g. the CMU Depository.
 

About ELIZA:

ELIZA was meant to be a hoax. The name  is that of the flower selling
girl in My Fair Lady who pretends to be more than she is.

Why is ELIZA interesting? For a brief explanation here is a quote from
the The New Hacker's Dictionary (also called the Internet Jargon File).

:ELIZA effect: /*-li:'z* *-fekt'/ [AI community] n. The tendency of
   humans to attach associations to terms from prior experience.
   For example, there is nothing magic about the symbol `+' that
   makes it well-suited to indicate addition; it's just that people
   associate it with addition.  Using `+' or `plus' to mean addition
   in a computer language is taking advantage of the ELIZA effect.

   This term comes from the famous ELIZA program by Joseph Weizenbaum,
   which simulated a Rogerian psychoanalyst by rephrasing many of the
   patient's statements as questions and posing them to the patient.
   It worked by simple pattern recognition and substitution of key
   words into canned phrases.  It was so convincing, however, that
   there are many anecdotes about people becoming very emotionally
   caught up in dealing with ELIZA.  All this was due to people's
   tendency to attach to words meanings which the computer never put
   there.  The ELIZA effect is a {Good Thing} when writing a
   programming language, but it can blind you to serious shortcomings
   when analyzing an Artificial Intelligence system.  Compare
   {ad-hockery}; see also {AI-complete}.

ELIZA is the ancient template of many current internet chat programs
("chatbots").  Via that connection, there is a subtle link to the Turing test.