Game sharing is coming to Steam
 

Steam is adding support for game sharing. The in-development Family Sharing feature is meant to allow "close friends and family members" to share their entire Steam libraries. Gamers will be able to grant access to as many as 10 different devices, although only one will be able to access the library at any given time. People playing shared games will be able to accumulate their own achievements and store saved games in the Steam Cloud. They won't be able to buy DLC add-ons for games they don't own, though.

http://techreport.com/news/25356/gam...oming-to-steam