The various grafitti in Portal can help you crack the code for the website www.aperturescience.com. Go to the website (it uses Shockwave Flash) and type in the command LOGIN.

The user name is CJOHNSON and the password is TIER3. Once you are in, you may type in HELP for a list of valid commands. Have fun!

Addition by PRO_GAMER_: You should also be able to login to aperturescience.com with the username GLADOS and the password PORTAL, as well.

Addition by red7ring: It actually doesn't matter what name you login with. You can login with any name you want. It's the password you insert that's important.

Addition by Spilled Salt: After logging in, type at the prompt "thecakeisalie" (minus the quotes). Doing so will unlock some secret messages for you to view.