Those who opted into the Steam client beta may see some interesting language alluding to game sharing in the "steamui_english" file of the latest update. It's suggested that Steam users will be able to set a shared game library – when the original owner logs into the game, the game sharer gets a prompt to quit out.
If you want to see the file for yourself, you'll need to be running the beta version of Steam – go into Steam settings and opt in, then restart the client. Then, you'll need to head into your public folder and open the "steamui_english" file in a text editor like Notepad. Final step: crack open a cold soda, sit back and feel good about being a computer genius.
http://www.joystiq.com/2013/06/19/rumor-steam-beta-update-file-suggests-game-sharing-is-coming/
This sounds pretty awesome, if true. There are a lot of old games I don't play that I'd be willing to share with friends, and there are some that I'd want to play but not sure about actually buying as well.