Steam announced a new "Family Sharing" feature today, and is accepting beta applications to test it. When it launches fully, close friends and family will be able to play one another's games, while still earning their own achievements and saving their individual progress through the cloud.
According to Valve's announcement, you'll be able to browse a family member's library and request authorization to download and play the game for yourself. You'll be able to share on up to 10 devices at a time, and sharing access means your partners will be able to look at your entire library. The FAQs do note that technical limitations like a third-party key might not allow for universal sharing, and a shared library can only be accessed by one user at a time.
You can signal your interest by joining the Family Sharing group. The beta will begin in mid-September.
The FAQs also address some nuts and bolts of how it will all work. As the lender, you'll always have access to your games. Playing when a borrower is already playing will give them a few minutes to either purchase it themselves, or quit for the time being. Borrowers will also have access to all of the lender's DLC, but they won't be able to purchase DLC for a game they don't own if the lender doesn't already have it. Region restrictions will remain in place as well, and Valve recommends you only lend to trusted family and friends, since your sharing could be revoked if one of your borrowers cheats with your games.
The feature is especially notable in light of the upcoming Xbox One launch. Family Sharing was one of Microsoft's planned features, but the reversal of its always-online policy meant the sharing feature was lost as well.