Ask HN: What software has made you go "wow?"

What software has really impressed you, which has made you realy think, "wow such an elegant solution!" or "wow how did they do that!?" etc? Can be any piece of software(games, webapps, dekstop adds, languages, everything)

some on my list:

  * Diablo (Blizzard's game...)
  * Firefox 3
  * First contact with Linux
  * Emacs
  * Python
  * YouTube (yes I'd gladly pay a monthly fee)
  * Wikipedia
  * Erlang (not so much the language but the philosophy)