Valve may be planning to let you share your games with friends on Steam. A few lines of code in the latest beta of the Steam client mention a "Shared game library" seem to indicate plans to let your friends borrow your games, with the ability to take them back whenever you'd like.
Three lines of code were spotted by a NeoGAF user (via Kotaku). The first simply references a shared game library, the second alerts the user that playing a game you own will cut off the borrower's use, and the third tells the borrower that the shared game is no longer available. It sounds like you'll only be able to lend out a game to one other user at a time, similar to physical copies.
We don't know when, or even if, Valve plans to implement this feature, just that the groundwork seems to be laid. We've contacted Valve and will update as more information becomes available.