Steam News announces the recently announced Steam Guard is out of beta, and now automatically available to all Steam users. Steam Support explains in detail how this can be used to safeguard your account, and here's a summary of how this works:

With Steam Guard enabled, anyone attempting to login as you from an unrecognized computer or browser must first provide additional, one-time authorization. A special access code will be sent to your contact email address, and this code must be entered into Steam before your first login on an unfamiliar computer is complete. You will also be notified if any login attempts from computers other than those you've authorized occur. Steam Guard essentially acts as a form of "User Rights Management," where you as the user have greater control over access to your stuff.