A game popular among hackers who code in the language APL (see write-only language and line noise). The objective is to see who can code the most interesting and/or useful routine in one line of operators chosen from APL's exceedingly hairy primitive set. A similar amusement was practiced among TECO hackers and is now popular among Perl aficionados.
Ken Iverson, the inventor of APL, has been credited with a one-liner that, given a number N, produces a list of the prime numbers from 1 to N inclusive. It looks like this:
(2 = 0 +.= T o.| T) / T <- iN
where "o" is the APL null character, the assignment arrow is a single character, and "i" represents the APL iota.