Valve has opened the doors to Steam's Family Sharing program. The game sharing feature was first introduced in September as a limited beta, but it should now be accessible to every Steam user. Sharing options can be managed through the Family section of Steam's Settings menu.
The limitations attached to the sharing program are fairly straightforward. Steam users can enable sharing for up to five accounts and 10 different devices; authorized accounts must use an authorized device to access shared games. Lenders are accountable for the actions of anyone borrowing their games, so it's probably best to limit your sharing to close family and friends.
As in the beta, the sharing program involves entire libraries rather than individual games. Some titles aren't compatible, though. Games that require third-party keys, accounts, or subscriptions can't participate in the sharing scheme. Region-locked releases can be shared, but only within their native regions.
Account owners always have priority over borrowers. If a library owner wants to play one of their games, anyone borrowing from that library will be given a few minutes to either quit or purchase a copy of the game. Shared games will be inaccessible to borrowers if the library is already in use, either by its owner or another approved account. There doesn't appear to be a mechanism for giving some borrowers priority over others.
The Family Sharing program sounds reasonable, and it's nice to see Valve effectively easing DRM restrictions on a wide swath of games. Let's hope this move encourages other game distribution services—Origin, I'm looking at you—to offer similar functionality. Thanks to Rock, Paper, Shotgun for the tip.