Spoiler free, fret not. We shall be elusive.
If you've played Portal, and you've been very observant, you'll have noticed some information the game didn't do anything with.
If so, then you'll know what to do when you re-visit the viral promo site of Aperture Science.
Type "LOGIN". The rest you can figure out. (Standard DOS commands).
And enjoy. Thanks to Graham for figuring it out. (And less thanks to the site, for ignoring when I tried "LOGIN" earlier today, robbing me of the glory).