I think Valve's done a great job with the ARG stuff they've been doing. They added the new achievement, which put radios in each level, and when the radios hit the fizzler, they play an SSTV sound file which encoded locations of dial-up BBSes, which broadcast ASCII art and stuff.
More info here:
There's just a bit more to Valve's recent Portal Update... | MetaFilter The most recent patch also added a new ending for Portal. You can check it on YouTube.