The Jargon File (version 4.4.7, 29 Dec 2003):

random numbers
 n.

    When one wishes to specify a large but random number of things, and the
    context is inappropriate for N, certain numbers are preferred by hacker
    tradition (that is, easily recognized as placeholders). These include the
    following:

    +-------------------------------------------------------------------------+
    |   |Long described at MIT as ?the least random number?; see also 23. This|
    |17 |may be Discordian in origin, or it may be related to some in-jokes   |
    |   |about 17 and ?yellow pig? propagated by the mathematician Michael    |
    |   |Spivak.                                                              |
    |---+---------------------------------------------------------------------|
    |23 |Sacred number of Eris, Goddess of Discord (along with 17 and 5).     |
    |---+---------------------------------------------------------------------|
    |   |The most random two-digit number is 37, When groups of people are    |
    |37 |polled to pick a ?random number between 1 and 100?, the most commonly|
    |   |chosen number is 37.                                                 |
    |---+---------------------------------------------------------------------|
    |   |The Answer to the Ultimate Question of Life, the Universe, and       |
    |42 |Everything (?what is 6 times 9?, correct in base 13). (This answer is|
    |   |perhaps not completely fortuitous; in Kabbalism, the true unspeakable|
    |   |name of God is said to have 42 characters.)                          |
    |---+---------------------------------------------------------------------|
    |69 |From the sexual act. This one was favored in MIT's ITS culture.      |
    |---+---------------------------------------------------------------------|
    |105|69 hex = 105 decimal, and 69 decimal = 105 octal.                    |
    |---+---------------------------------------------------------------------|
    |666|In Christian mythology, the Number of the Beast.                     |
    +-------------------------------------------------------------------------+

    For further enlightenment, study the Principia Discordia, The Hitchhiker's
    Guide to the Galaxy, The Joy of Sex, and the Christian Bible (Revelation
    13:18). See also Discordianism or consult your pineal gland. See also 
    for values of.