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.