Sharing games you have already purchased has always been pretty easy when it came to consoles, you would just take the disc or cartridge over a friends house. When it comes to PC gaming things are a little bit harder because of licensing issues and DRM. This might change very soon as the latest beta client of Steam has a short section of code that mentions a “Shared Game Library”.
The code snippet is below…
“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.”
So it looks like possibly in the future you will be able to share a game you have purchased from a friend. You will only be able to share a game with one friend and that user can only play the game when you are not playing it. If you want to play the game and a friend is playing it Steam will send them a notice that you want to play and that they should stop.
Not sure when this feature will be available, if at all, but we can hope!