Steam has rolled out its Family Sharing feature to all users today following its beta release. As the name implies, it allows users to share their game libraries with others.
To enable the feature, first set up Steam Guard if you haven't done so already and go to Steam > Settings > Family. From here, you can authorize your computer and users to share. A maximum of 10 devices can be authorized simultaneously for up to five separate accounts.
Valve has specifics about Family Sharing over here, if you'd like to read up on the literature.