Steam Guard, Valve’s new account security system announced last week at the Game Developers Conference, is in beta beginning today. The new account security system is available to anyone who is willing to take it for a test run. To take part in the beta, you will have to verify your email with steam, which allows you to tie your Steam account to your computer. If you want to use your Steam account on another computer, you'll have to get a verification code, which will be sent to your email address on file.
The sign-up for the Steam Guard beta is available via the beta participation section in your Steam account settings.
Any attempts to login for the first time on a computer elsewhere will need a verification code, which will be sent to your email address. To access the beta, select Steam Guard from the beta participation section in your Steam account settings.
Valve's Gabe Newell challenged the public to test the Steam Guard feature out by hacking his account. He gave out his username email and password and challenged everyone to try and get into his Steam account.
His email is firstname.lastname@example.org, and his password is moolyftw, for those of you out there that want to give it a shot..
Source: PC Gamer