http://www.escapistmagazine.com/news/view/98776-Crazy-Valve-ARG-Teases-Portal-2
Apparently the new Portal update has revealed a bunch of cryptic multi-layered clues, information and what seems to be an overall incredibly well thought-out scheme from Valve to properly build up to something big coming in the nearest days.
From hidden images in sound files, to morse code, and all the way to an old 80s modem set up near Valve HQ, transmitting ASCII pictures of test chambers, Vortigaunts and more.
Whenever the community cracks/decodes one layer of information, more transcend from it. It just keeps getting more and more interesting.
It better be good, Valve. I'm thinking new hats for TF2/L4D2? Episode 3/Portal 2?
Discuss.
EDIT:
Summary of things discovered so far