42

42 is the Answer to Life, the Universe, and Everything.

Saying that something does not equal 42 is one of the gravest of insults.

42 must never be divided by 0.

42 is of great traditional importance to CTY; it is involved in things such as 42-second hugs, 42-person orgies, 42-player Blammo games, etc.

42 is the number of roads one must take in his/her life life time.

Actually, 42 is the answer to the question "What do you get when you multiply 6 times 9?" This is to people who actually read The Restaraunt at the End of the Universe.

  Actually, that's not really true (can't say more without giving spoilers). - Ryan

See The Hitchhiker's Guide to the Galaxy.

It is worth noting that 6 times 9 does indeed equal 42, in base 13.