Valve has confirmed that it has unlocked Steam 'Family Sharing' for all account holders.
Family Sharing essentially allows users to share their library of Steam games with other account holders by authorizing a PC as a shared device. The feature was previously launched in beta form in September last year.
With the new functionality, a user can authorize up to ten devices for game sharing, which allows anyone who signs into said devices to access and play games in the shared library, while still being able to create their own save files and earn their own achievements.
It does come with certain restrictions, however; when the primary account holder logs in, any user playing a shared game will be given a few minutes to "either purchase the game or quit playing", warns Valve. Additionally, only one shared user may be playing at once (priority is given to the user currently logged in). A full Q&A is here.