Valve's new account security system, Steam Guard, is now in beta and available to anyone who wants to give it a try. Once you've verified your email address you can tie your Steam account to your computer. 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.
Head of Valve, Gabe Newell, is so confident in the new system that he recently gave out his username and password and challenged the public to gain access to his account. It's firstname.lastname@example.org, and moolyftw, if you want to give it a go.