The graffiti in the game are clues for the ApertureScience.com web site. Go to the site and enter LOGIN as a command. Enter CJOHNSON as a username and TIER3 as a password. After logging in, type HELP for a list of commands.

In addition to logging in as "CJOHNSON" (Chell), you can login as anyone desired by typing a username longer than two characters and using PORTAL as a password.