A few lines of code in the latest Steam client beta suggests that Steam users may be able to share their games with other users on their friends list.
According to a post from Neogaf by user "Grief.exe", the discovered lines of code suggest that users may eventually be able to loan out games in their library and take them back at any time. The relevant code of text from the client are as follows:
"SteamUI_JoinDialog_SharedLicense_Title" "Shared game library"
"SteamUI_JoinDialog_SharedLicenseLocked_OwnerText" "Just so you know, your games are currently in use by %borrower%. Playing now will send %borrower% a notice that it's time to quit."
"SteamUI_JoinDialog_SharedLicenseLocked_BorrowerText" "This shared game is currently unavailable. Please try again later or buy this game for your own library."
There is currently no word on how Valve will handle this among third-party publishers. For more news on Steam, check out GameSpot's recent coverage.