A NeoGAF user poking around the steamui_english file has found possible evidence indicating a Steam feature allowing you to share your games with other people.
It looks like you can add games you own to a shared library that you can grant other people access to. You can’t play a game that someone is already playing, whether you’re the owner or the borrower, but when the owner wants to play a game that’s being shared it sends the borrower a message to log off.
This feature, if implemented, could have a major impact on PC gaming. Publishers may not take too kindly to players sharing their game licenses with one another. Likely sharing will be limited to certain games that have been authorized as sharable by the publisher. Still, any additional level of consumer control over digital games is a step in the right direction.