In September 2013, Steam has announced that its Family Sharing feature was going to be open for beta.
Now, it is out of beta!
Thanks to Family Sharing, up to ten devices and five accounts can access each other’s libraries and share games without disrupting their own save files and achievements. All you need to do is request for authorization from someone in your family and you will be able to download and share their entire library of games.
However, only one person can be online on a particular game at a time, meaning you cannot play multiplayer games with family members by sharing a library. Also, in case you thought you had found a way around region restriction, region-restricted content cannot be shared across regions.