This number:

  4856507896573978293098418946942861377074420873513579240196520736 6869851340104723744696879743992611751097377770102744752804905883 
  1384037549709987909653955227011712157025974666993240226834596619 6060348517424977358468518855674570257125474999648219418465571008 
  4119086259716947970799152004866709975923596061320725973797993618 8606316914473588300245336972781813914797955513399949394882899846 
  9178361001825978901031601961835034344895687053845208538045842415 6548248893338047475871128339598968522325446084089711197712769412 
  0795862440547161321005006459820176961771809478113622002723448272 2493232595472346880029277764979061481298404283457201463489685471 
  6908235473783566197218622496943162271666393905543024156473292485 5248991225739466548627140482117138124388217717602984125524464744 
  5055834628144883356319027253195904392838737640739168912579240550 1562088978716337599910788708490815909754801928576845198859630532 
  3823490558092032999603234471140776019847163531161713078576084862 2363702835701049612595681846785965333100770179916146744725492728 
  3348691600064758591746278121269007351830924153010630289329566584 3662000800476778967984382090797619859493646309380586336721469695 
  9750279687712057249966669805614533820741203159337703099491527469 1835659376210222006812679827344576093802030447912277498091795593 
  8387121000588766689258448700470772552497060444652127130404321182 610103591186476662963858495087448497373476861420880529443

This is a 1811-digit illegal prime number, which when unpacked into binary, becomes a compressed ELF executable which will decrypt DVDs. This was one of many ways used to enable people who had legally purchased a DVD with copy protection to play it on Linux. Here's the story behind finding the prime: https://web.archive.org/web/20070223075434/http://asdf.org/~...

Aside from that, the qrpff perl scripts that became t-shirts (https://web.archive.org/web/20011221024307/http://www.copyle...) were another fun and illegal way to point out the stupidity of the DMCA. But they aren't so pretty ;)