Steam will soon let users share their library of games with up to 10 authorized devices via Steam Family Sharing. The new feature announced today will go into beta in "about a week," according to its Steam Group.
Users can share their libraries with one another by authorizing devices, either remotely by email or in-person via Steam's settings. Once that device is authorized, one friend at a time can access, download, and play any supported game from the lender's library, unlocking achievements and saving progress the same as if the friend owned it.
But Steam Family Sharing is meant to share libraries as a whole, not individual games. Therefore if a "borrowing" user is playing a game when the owner starts playing (the same game or any other in the shared library), the borrower will be given a few minutes to save his or her progress and either buy the game or quit.
Theoretically, you and a friend could each play the other's library as long as you want. But as soon as one of you wants to play from your own, the other will get booted out.
Also note that not all Steam games are eligible: those that require an external CD key, account, or subscription are off the table. And all region locks will remain in place, no matter who owns what.
Despite the restrictions, it's a promising step for more flexible digital ownership. Join Steam Family Sharing's official group for a chance at a beta invite.